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The exact etiology of clinical cases of acute liver failure is difficult to ascertain and it is likely that various co-morbidity factors play a role. For example, epidemiological evidence suggests that coexistent hepatitis C virus (HCV) infection increased the risk of acetaminophen-induced acute liver injury, and was associated with an increased risk of progression to acute liver failure. However, little is known about possible mechanisms of enhanced acetaminophen hepatotoxicity in HCV-infected subjects. In this study, we tested a hypothesis that HCV-Tg mice may be more susceptible to acetaminophen hepatotoxicity, and also evaluated the mechanisms of acetaminophen-induced liver damage in wild type and HCV-Tg mice expressing core, E1 and E2 proteins. Male mice were treated with a single dose of acetaminophen (300 or 500 mg/kg in fed animals; or 200 mg/kg in fasted animals; i.g.) and liver and serum endpoints were evaluated at 4 and 24h after dosing. Our results suggest that in fed mice, liver toxicity in HCV-Tg mice is not markedly exaggerated as compared to the wild-type mice. In fasted mice, greater liver injury was observed in HCV-Tg mice. In fed mice dosed with 300 mg/kg acetaminophen, we observed that liver mitochondria in HCV-Tg mice exhibited signs of dysfunction showing the potential mechanism for increased susceptibility., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Identification of carcinogenic activity is the primary goal of the 2-year bioassay. The expense of these studies limits the number of chemicals that can be studied and therefore chemicals need to be prioritized based on a variety of parameters. We have developed an ensemble of support vector machine classification models based on male F344 rat liver gene expression following 2, 14 or 90 days of exposure to a collection of hepatocarcinogens (aflatoxin B1, 1-amino-2,4-dibromoanthraquinone, N-nitrosodimethylamine, methyleugenol) and non-hepatocarcinogens (acetaminophen, ascorbic acid, tryptophan). Seven models were generated based on individual exposure durations (2, 14 or 90 days) or a combination of exposures (2+14, 2+90, 14+90 and 2+14+90 days). All sets of data, with the exception of one yielded models with 0% cross-validation error. Independent validation of the models was performed using expression data from the liver of rats exposed at 2 dose levels to a collection of alkenylbenzene flavoring agents. Depending on the model used and the exposure duration of the test data, independent validation error rates ranged from 47% to 10%. The variable with the most notable effect on independent validation accuracy was exposure duration of the alkenylbenzene test data. All models generally exhibited improved performance as the exposure duration of the alkenylbenzene data increased. The models differentiated between hepatocarcinogenic (estragole and safrole) and non-hepatocarcinogenic (anethole, eugenol and isoeugenol) alkenylbenzenes previously studied in a carcinogenicity bioassay. In the case of safrole the models correctly differentiated between carcinogenic and non-carcinogenic dose levels. The models predict that two alkenylbenzenes not previously assessed in a carcinogenicity bioassay, myristicin and isosafrole, would be weakly hepatocarcinogenic if studied at a dose level of 2 mmol/kg bw/day for 2 years in male F344 rats; therefore suggesting that these chemicals should be a higher priority relative to other untested alkenylbenzenes for evaluation in the carcinogenicity bioassay. The results of the study indicate that gene expression-based predictive models are an effective tool for identifying hepatocarcinogens. Furthermore, we find that exposure duration is a critical variable in the success or failure of such an approach, particularly when evaluating chemicals with unknown carcinogenic potency., (Published by Elsevier Inc.)

Bayesian networks for quantifying linkages between genes were applied to detect differences in gene expression interaction networks between multiple doses of acetaminophen at multiple time points. Seventeen (17) genes were selected from the gene expression profiles from livers of rats orally exposed to 50, 150 and 1500 mg/kg acetaminophen (APAP) at 6, 24 and 48 h after exposure using a variety of statistical and bioinformatics approaches. The selected genes are related to three biological categories: apoptosis, oxidative stress and other. Gene interaction networks between all 17 genes were identified for the nine dose-time observation points by the TAO-Gen algorithm. Using k-means clustering analysis, the estimated nine networks could be clustered into two consensus networks, the first consisting of the low and middle dose groups, and the second consisting of the high dose. The analysis suggests that the networks could be segregated by doses and were consistent in structure over time of observation within grouped doses. The consensus networks were quantified to calculate the probability distribution for the strength of the linkage between genes connected in the networks. The quantifying analysis showed that, at lower doses, the genes related to the oxidative stress signaling pathway did not interact with the apoptosis-related genes. In contrast, the high-dose network demonstrated significant interactions between the oxidative stress genes and the apoptosis genes and also demonstrated a different network between genes in the oxidative stress pathway. The approaches shown here could provide predictive information to understand high- versus low-dose mechanisms of toxicity.

Toxicity studies were performed by exposing F344/N rats and B6C3F1 mice (10 animals per sex per species per group) to transient-free, linearly polarized 60 Hz magnetic fields for 8 weeks. Targeted magnetic fields strengths used were 0 gauss (G; sham control fields did not exceed 0.001 G), 0.02 G, 2 G, and 10 G. Exposure was whole-body and continuous for 18.5 hr per day, 7 days per week. An additional group of rats and mice was exposed intermittently (1 hr on/1 hr off) to 10 G fields for the same period of time. Endpoints evaluated included morbidity, mortality, gross pathology, histopathology, body/organ weights, clinical chemistry (rats only), and hematology (rats only). All mice and all male rats survived until the end of the study. One female rat (2-G exposure group) died during Week 7 of the study; the death was not attributed to magnetic field exposure. In both studies, the mean body weight gains of exposed animals were similar to those of the respective controls. There were no gross, histological, hematological, or biochemical lesions attributed to magnetic field exposure. Statistically significant increases in liver weight and liver to body weight ratio occurred in female rats of all exposure groups but only at the termination. These data suggest that, for the variables evaluated in these studies, an 8-week exposure to linear-polarized, transient-free 60 Hz magnetic fields at field intensities of up to 10 G is not associated with significant toxicity in F344/N rats and B6C3F1 mice. Furthermore, there was no toxicity observed in animals receiving intermittent (1 hr on/1 hr off) exposures to 10-G fields. A 2-year study in F344/N rats and B6C3F1 mice is nearing completion of the in-life phase without overt toxicity in any exposed group. It is premature, however, to make any prediction concerning the possible influence of exposure to 60 Hz magnetic fields on cancer rates.

Lindane (gamma-1,2,3,4,5,6-hexachlorocyclohexane), a widely used insecticide, may be found at low concentrations in the human diet. Male B6C3F1 mice given lindane daily at doses of 20 and 40 mg/kg body wt by gavage in corn oil for 3 days had suppressed bone marrow cellularity, erythrocyte precursors, granulocyte-macrophage progenitor cells (CFU-GM), and residual progenitor cell damage, which could be demonstrated by two whole-body irradiations (WBI) at 200 rads. Lindane exposure for 10 consecutive days at doses of 0, 10, or 20 mg/kg did not cause clinical abnormality or changes in body weights, but there were dose-dependent decreases in marrow cellularity, in more pluripotent stem cells and in committed CFU-GMs, which returned to control values by 4 weeks. These mice were then subjected to two 100-rad exposures of WBI at 4 and 9 weeks following cessation of lindane treatment. This level of irradiation caused only a transient drop in number of marrow progenitor cells. Control and lindane-exposed mice were examined at 1 and 6 weeks following the last irradiation, which was 10 and 15 weeks following the final lindane exposure. The lindane-exposed mice had lower progenitor cell numbers and slower recovery from the irradiation. These results indicate that lindane has significant myelotoxicity in mice and short-term lindane exposure can induce residual progenitor cell damage that can be demonstrated by subsequent irradiation.

Melamine (2,4,6-triamino-s-triazine) was administered in the diet to F344 rats or B6C3F1 mice for 13 weeks (subchronic) or for 103 weeks (chronic) to determine its toxicologic profile, including carcinogenic potential in the chronic study. The dose levels of melamine in the subchronic studies ranged from 750 to 18,000 ppm for rats, and 6000 to 18,000 ppm for mice. In the chronic studies the dose levels of melamine were 2250 or 4500 ppm for male rats and mice of each sex, and 4500 or 9000 ppm for female rats. In these studies, compound-related lesions were observed in the urinary tract. Most noticeable was the development of uroliths (urinary bladder stones), which occurred at a greater frequency in males than females of either species. Increased incidences of urinary bladder stones and hyperplasia of the bladder epithelium were observed at 13 weeks in male rats fed diets containing melamine. In the chronic study, transitional-cell carcinomas in the urinary bladder of male rats occurred at a significantly (p less than or equal to 0.016) higher incidence in the 4500 ppm (high dose) group (8/49) than in the controls (0/45). Seven of the eight male rats with transitional-cell carcinomas of the urinary bladder also had bladder stones. There was a statistically significant association (p less than or equal to 0.001) between bladder stones and bladder tumors in male rats fed melamine (4500 ppm). Urinary bladder tumors were not observed in the low-dose (2250 ppm) male rat group, while bladder stones were observed in one rat in this group. In the female rat chronic study, chronic inflammation of the kidney was observed at an increased incidence (relative to controls) in both the low (4500 ppm) and high (9000 ppm) dose groups. Ulceration of the bladder epithelium was observed in male and female mice in the 13-week study. The distribution of these toxic lesions was not correlated statistically with the distribution of urinary bladder stones. Acute and chronic inflammation and epithelial hyperplasia of the urinary bladder were found in increased incidence in dosed male mice (2250 and 4500 ppm) in the chronic study. In addition, a high incidence of urinary bladder stones was observed in dosed male mice relative to controls. However, there was no evidence of bladder tumor development in this species.

Blood cells are derived from a multitiered system of progenitor stem cells that lose their capacity for proliferation and self-renewal as they continue along pathways of differentiation. Since these hematopoietic events can be readily monitored in vivo and in vitro in the mouse, we have utilized this system to examine altered cellular differentiation associated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity. Progenitor cells were suppressed following acute exposure of mice to TCDD at doses as low as 1.0 micrograms/kg body wt. In vitro studies demonstrated that myelotoxicity occurs by a direct inhibition of proliferating stem cells. Genetic studies indicated that the myelotoxic responses to TCDD, both in vivo and in vitro, segregate with the Ah locus. In addition, the in vitro myelotoxicity of various polyhalogenated aromatic hydrocarbon congeners correlated with their previously reported ability to induce hepatic microsomal enzyme activity and to bind to an intracellular receptor for TCDD. TCDD was also found to bind specifically to bone marrow cells from Ah-responsive, but not nonresponsive mice, indicating that bone marrow cells possess a specific receptor for TCDD. These data indicate that the myelotoxic response to TCDD is regulated by the Ah receptor present in the target tissue and demonstrates the utility of this system for examining the cellular and molecular events associated with the toxicity of polyhalogenated aromatic hydrocarbons, the prototype for which is TCDD.

F344 rats and B6C3F1 mice of both sexes were exposed by inhalation to 0, 1, and 3 ppm methyl isocyanate (MIC) for 4 consecutive days (6 hr/day) followed by a recovery period of 91 days. Five mice and rats/sex/group except the 3 ppm group (5 rats/sex on Day 7 and 2 males on Day 28) were killed on Days 7, 28, 49, and 91 after the exposure and examined histopathologically. Forty-nine of 56 male rats, 51 of 56 female rats, and 1 of 56 male mice in the 3 ppm group died by 28 days; early death animals were also examined histologically. Exposure-related changes occurred in rats and mice of both sexes in the 3 ppm group only. Lesions of the nasal cavity in rats and mice were characterized by regeneration of the olfactory and respiratory epithelia secondary to epithelial erosion. By Day 28 the olfactory and respiratory epithelia in mice appeared normal, while in rats incomplete regeneration of the olfactory epithelium was still present. Regeneration of the respiratory epithelium in the trachea of rats occurred in the 3 ppm group and the epithelium appeared to return to normal by Day 28. Lung lesions in rats consisted of mural and/or intraluminal fibrosis secondary to extensive erosion of the respiratory epithelium in the major bronchi to the terminal bronchioles. Acute inflammation of the small airways, occasional hyaline membranes of alveolar walls, and pulmonary atelectasis were also seen. Alveolar fibrosis was observed in rats found dead from Day 14 on and in male rats killed on Day 28. Atrophy of the thymus and spleen, atrial thrombosis of the heart, and hepatocellular necrosis were frequently seen in rats dying following MIC exposure. The lung lesions in mice were qualitatively similar to those in rats, but were restricted to the major bronchi. Minimal intraluminal or mural fibrosis was still present in mice on Day 91. In a separate study, a single 6-hr exposure of five male rats to 3 ppm MIC was followed by a recovery period of 7 days. The lesions of the respiratory system were essentially the same as those in the 3 ppm group killed on Day 7 after the 4-day repeated exposure of MIC, but the alveolar lesions were more severe.

Arsine gas is a potent hemolytic agent. Concern about semiconductor workers prompted an in-depth study of arsine at the National Institute of Environmental Health Sciences to determine the hematopoietic effects of prolonged exposure to this gas. Female B6C3F1 mice were exposed by inhalation to 0, 0.5, 2.5, and 5 ppm arsine, 6 hr/day for 14 days. Body weights of exposed mice were comparable to those of controls, but a marked, concentration-related splenomegaly was observed. Higher level arsine exposure produced statistically significant decreases in red blood cells, hematocrit and hemoglobin, with increases in white blood cell counts and mean corpuscular volume of red blood cells. Erythropoiesis as measured by quantitation of erythroid precursors in culture revealed a marrow reduction of colony-forming unit erythroids/femur cells for all treated groups on Day 3 postexposure and only at the 5 ppm dose group on 24 days postexposure, while splenic erythropoiesis increased at higher concentrations of arsine. There was no alteration in bone marrow cellularity and a less significant effect on granulocyte-macrophage progenitors. A 12-week study of arsine at 0, 0.025, 0.5, and 2.5 ppm (6 hr/day) by inhalation showed similar effects on hematopoiesis in mice. In conclusion, arsine exposure at low concentrations produces a stress on the hematopoietic system characterized by hemolysis, which persists for a prolonged period following exposure.

Female Golden Syrian hamsters, F-344 rats, Swiss CD-1 mice, and B6C3F1 hybrid mice were exposed 6 hr/day, 5 days/week to carcinogenic levels of vinyl chloride (VC) for 6, 12, 18, or 24 months (rats and hamsters only). Other groups of rodents were held for 6 or 12 months and then exposed for 6 or 12 months. At the end of the study the incidence of VC-induced neoplasms was compared in each of the groups to assess the effects of duration of exposure and age at the start of exposure on carcinogenicity of VC. In rats, with early initial exposure, hemangiosarcomas, hepatocellular carcinomas, and mammary gland carcinomas occurred with increasing incidence with longer exposure duration. Rats held for 6 months before exposure developed VC-related neoplasms, while rats held 12 months before the start of exposure failed to show a significantly increased incidence of these neoplasms. In hamsters, hemangiosarcomas, mammary gland carcinomas, gastric adenocarcinomas, and skin carcinomas resulted from VC exposure. The highest incidence of malignant neoplasms occurred in hamsters exposed for the first 12 months, whereas exposure begun after 12 months of age did not cause neoplasms. In both strains of mice, VC exposure during the first 6 months of the experiment induced a high incidence of hemangiosarcomas and mammary gland carcinomas. Swiss mice also developed lung carcinomas after only 6 months of exposure. In all three rodent species an initial 12 month exposure to VC was adequate to detect its carcinogenic potential, but the shortened survival of VC exposed mice and hamsters precluded a meaningful comparison with longer periods of exposure. Exposures were most effective when started early in life.

Chronic toxicity studies of propylene were conducted by exposing groups of 50 F344/N rats and 50 B6C3F1 mice of each sex in chambers to air containing the chemical in concentrations of 5000 and 10,000 ppm, 6 hr per day, 5 days per week, for 103 weeks. Groups of 50 rats and 50 mice of each sex in similar chambers received clean air only on the same schedule and served as controls. Survival and mean body weights of exposed and control male and female rats and mice were similar. In exposed rats, increased incidences of nonneoplastic lesions were observed in the nasal cavity. These consisted of epithelial hyperplasia in female rats exposed to the high concentration, and squamous metaplasia in female rats exposed to both concentrations and in male rats exposed to the low concentration. In addition, inflammatory changes characterized by an influx of lymphocytes, macrophages, and granulocytes into the submucosa and by granulocytes into the lumen occurred in male rats of both exposure groups. There was no evidence of nasal cavity lesions in exposed mice, suggesting a species difference in sensory irritation to propylene. There were no treatment-related increases or decreases in tumor incidence in the exposed groups relative to controls for either rats or mice. These data suggest that inhaled propylene induces signs of nasal cavity toxicity in rats but not in mice, and that the chemical is not carcinogenic to either species at the concentrations tested.

11-Aminoundecanoic acid, the monomer of nylon 11, was toxic to the urinary tract of both male and female B6C3F1 mice and Fischer 344 rats, when administered in the diet at 7500 or 15 000 ppm for 103-104 weeks. Dose-related effects included a decrease in mean body weight gain and in survival for male rats and for mice of each sex; increased incidence of hyperplasia of the transitional epithelium of the kidney in rats of each sex; increased incidence of calcification of the kidney in the female rats; increased incidence of hyperplasia of the urinary bladder in male rats; and mineralization of the kidney in mice of each sex. Transitional cell carcinomas of the urinary bladder of the male rat occurred with increased frequency in the high-dose group (control, 0/48; low-dose, 0/48; high-dose, 7/49). Additional evidence for carcinogenicity in the male rat was seen in the liver, where an increased frequency of neoplastic nodules was found in the treated animals (controls, 1/50; low-dose, 9/50; high-dose, 8/50). Therefore, under the conditions of these studies, 11-aminoundecanoic acid was carcinogenic for male Fischer 344 rats, inducing transitional cell carcinomas in the urinary bladder and neoplastic nodules in the liver. The test chemical was not demonstrated to be carcinogenic for female Fischer 344 rats or for B6C3F1 mice of either sex.

Adult B6C3F1 male mice were given water containing 3, 15, and 75 ppm mercury (as mercuric chloride) for 7 weeks. There were dose-related increases in blood and kidney mercury levels but only the former showed a time-dependent change. Mercury was not detected in any of the lymphoid organs except for the spleen. There was no mortality and only minimal histological changes occurred in kidneys of dosed mice. Nonspecific toxicity occurred at the 75 ppm dose level, consisting of small differences in body and organ weights, hematological changes, and general enzyme inhibition in the bone marrow and spleen. However, there were specific immunotoxic and biochemical alterations in lymphoid organs of mice treated at the lower doses of mercury. The immunological defects were consistent with altered T-cell function as evidenced by decreases in both T-cell mitogen and mixed leukocyte responses. There was a particular association between the T-cell defects and inhibition of thymic pyruvate kinase, the rate-limiting enzyme for glycolysis. The differences in the pattern of enzyme responses among lymphoid organs implied that two mechanisms of mercury toxicity were operative--one at high concentrations that caused physicochemical enzyme inhibition and another at low concentrations that caused indirect enzyme inhibition.

Concern over the potential adverse health effects of chemically contaminated groundwater has existed for many years. In general, these studies have focused on retrospective epidemiological studies for cancer risk. In the present studies, immune function was monitored in female B6C3F1 mice exposed to a chemical mixture in drinking water for either 14 or 90 days. The mixture consisted of 25 common groundwater contaminants frequently found near toxic waste dumps, as determined by EPA surveys. None of the animals developed overt signs of toxicity such as body or liver weight changes. Mice exposed to the highest dose of this mixture for 14 or 90 days showed immune function changes which could be related to rapidly proliferating cells, including suppression of hematopoietic stem cells and of antigen-induced antibody-forming cells. Some of these responses, e.g., granulocyte-macrophage colony formation, were also suppressed at lower concentrations of the chemical mixture. There were no effects on T cell function or T and B cell numbers in any of the treatment groups. Altered resistance to challenge with an infectious agent also occurred in mice given the highest concentration, which correlated with the immune function changes. Paired-water studies indicated that the immune effects were related to chemical exposure and not to decreased water intake. These results suggest that long-term exposure to contaminated groundwater may represent a risk to the immune system in humans.

The effect of chrysotile asbestos exposure on bone marrow and immune parameters was examined in mice at 2, 12, and 26 weeks following a 3-day inhalation exposure. Ultrastructural examination revealed that the fibers were deposited primarily at alveolar duct bifurcations within the centriacinar region of the lung. Histological pulmonary changes were minimal, but by 26 weeks early asbestosis characterized by clusters of macrophages and minimal fibrosis were present in the centriacinar region of the lung. Lymphoproliferative responses, antibody levels, and number of plaque forming cells were not significantly altered in exposed mice. Pulmonary macrophages, but not peritoneal macrophages, showed evidence of activation in the chrysotile-exposed mice at 26 weeks following exposure. The most striking change was the depression of the number of bone marrow pluripotent stem cells (CFU-S) and marrow granulocyte macrophage progenitors (CFU-GM) which were lower at all three postexposure examinations. It is felt that the depression of bone marrow progenitors in asbestos-exposed mice may have relevance to the leukopenia reported in workers with occupational history of asbestos exposure.

Six- to seven-week-old female B6C3F1 mice were administered a total of 0, 20, 40, or 80 mg/kg of ochratoxin A (OCT A) ip on alternate days over an 8-day period. Twenty-four hours following the final dose, histopathology, bone marrow, and macrophage parameters were assayed. There was a dramatic dose related decrease in thymic mass with the mean thymus weight of the high dose animals being only 33% of controls. Histologic evidence of nephrotoxicity was minimal and restricted to the inner cortex. Myelotoxicity was present as evidenced by bone marrow hypocellularity, decreased marrow pluripotent stem cells (CFU-S), granulocyte-macrophage progenitors (CFU-GMs), and decreased 59Fe uptake in marrows and spleens of exposed mice. Peritoneal macrophages from sc as well as ip injected mice demonstrated increased phagocytic capacities and increased capacity to inhibit tumor cell growth. These alterations in bone marrow cells and macrophages suggest myelotoxicity is an additional potential hazard of OCT A exposure.

There is continuing concern about the role of irritation in cancer development. Methyl bromide, a widely used fumigant and known irritant reported to cause forestomach carcinomas in rats, was dissolved in peanut oil and given by gavage at 50 mg/kg body wt to Wistar rats five times per week for 13 to 25 weeks. Starting at Week 13, methyl bromide administration was discontinued for half of the methyl bromide-treated rats (stop treatment group). After that, rats from both the continuous treatment and stop treatment groups were terminated at 4-week intervals to follow the progression of the stomach lesions. Forestomach lesions were not found in control rats receiving peanut oil and killed at 13 or 25 weeks. At 13 weeks the forestomachs from rats receiving methyl bromide were contracted and adherent to the liver and spleen. Inflammation, acanthosis, fibrosis, and a high incidence of pseudoepitheliomatous hyperplasia were found microscopically in treated animals. At 25 weeks, 100% of the rats receiving methyl bromide continuously had hyperplastic lesions of the forestomach which were more severe than those at 13 weeks. Evidence of malignancy was seen in one rat and the lesion was considered a very early carcinoma. In the stop treatment group that received methyl bromide for 13 weeks, there was regression of the stomach lesions, but at the 12-week final sacrifice, adhesions, fibrosis, and mild acanthosis remained. This study illustrates the need for regression experiments for complex forestomach lesions in rodents, especially when an irritating chemical is given by gavage.

Female B6C3F1 mice plus male and female Fischer 344/N rats were gavaged with allyl isovalerate (AIV) in corn oil at 0, 31, 62, or 125 (mice) and 0, 31, 62, 125, or 250 (rats) mg/kg body weight for five daily exposures per week for a 2-week period. Hematologic, immunologic, and histopathologic studies were performed 48 to 72 hr following the final treatment. AIV exposure had no effect on hematology or bone marrow cellularity in mice or rats. AIV exposure at 250 mg/kg was toxic to rats causing reduced weight gain and hepatotoxicity. In vivo and in vitro studies revealed that pluripotent hematopoietic stem cells (CFU-S) and granulocyte-macrophage progenitors (CFU-GM) in the bone marrow were decreased in the treated mice. Hematopoietic suppression was correlated with the reduction in the hexose monophosphate shunt metabolism of bone marrow cells but the Embden-Meyerhof pathway and tricarboxylic acid pathway enzymes did not appear to be affected. Examination of host resistance following Plasmodium and Listeria challenge did not demonstrate significant differences between treated and control mice, nor were there other effects on the immune system. This suggests that the myelotoxic effects were minimal and of a degree that would not alter host resistance.

Male and female mice were exposed by inhalation to 0, 3, 10, and 30 ppm methyl isocyanate for 2 hr followed by a 91-day recovery period. Sixteen of 80 (20%) male mice in the 30-ppm group died following exposure with seven deaths occurring in the first 24 hr. Two female mice died, one each in the 30- and 10-ppm concentration groups. Five mice/sex/group were examined at 2 hr and at 1, 3, 7, 14, 28, 49, and 91 days following exposure. Treatment-related changes were restricted to the respiratory system. At 30 ppm there was extensive necrosis and erosion of the respiratory and olfactory epithelia in the nasal cavity. Severe necrosis and epithelial erosion were also found in the trachea and main bronchi. Epithelialization occurred rapidly and most of the denuded turbinates were covered by flattened epithelium by Day 3. On the turbinates, recovery was essentially complete with the exception of small areas in the olfactory epithelia where lesions were present in the male mice through Day 91. In the trachea and major bronchi, fibrin and cellular debris were present in the airways, which in some cases had organized and formed intraluminal fibrotic projections. These projections became covered by respiratory epithelium. The intraluminal projections and bronchial fibrosis persisted to Day 91. In the males, where the fibrosis was more severe, chronic alveolitis and atelectasis were found. In mice exposed to 3 or 10 ppm, there appeared to be complete recovery. These inhalation studies indicate that methyl isocyanate exposure of mice at or near lethal concentrations causes reversible lesions in the nose and persistent intraluminal and mural fibrosis of the major bronchi.