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As part of the National Contaminant Biomonitoring Program (NCBP, formerly a component of the National Pesticide Monitoring Program), the U.S. Fish and Wildlife Service periodically determined concentrations of organochlorine chemical residues and elemental contaminants in freshwater fish collected from a nationwide network of stations. In late 1986 and early 1987, the last time the network was sampled, a total of 319 composite fish samples were collected from 97 NCBP stations. The samples were analyzed for residues of organochlorine chemicals and the elements As, Cd, Cu, Hg, Pb, Se, and Zn. The mean concentration of total DDT and its homologs (p,p'-constituents) declined from 1984 to 1986, thus continuing a trend that began in 1970. The most persistent DDT homolog (p,p'-DDE) was detected at all stations sampled in 1986, and averaged 74% of total DDT residues, up from 70% in 1974-1979 but essentially unchanged from 1984. Collectively, these findings indicated a low rate of influx and continued weathering of DDT in the environment; nevertheless, DDT concentrations in fish from some stations in the South remained high enough to constitute a threat to piscivorous wildlife. Residues of polychlorinated biphenyls (PCBs) also remained widespread, but a significant downward trend in total PCB concentrations and incidence was evident, and PCB mixtures containing early eluting components were present at fewer stations than in the past. PCB concentrations were generally highest in fish from the industrialized rivers of the Northeast and Midwest and from the Great Lakes. Concentrations of toxaphene also declined, as did its incidence, from 88% of the stations sampled in 1980-1981 to 64% in 1986; however, analytical problems with the 1986 samples may have contributed to the latter. The risks represented by PCBs and toxaphene could not be evaluated on the basis of our data. Among cyclodiene insecticides, dieldrin and chlordane-related residues were the most widespread. Mean concentrations of dieldrin declined through 1986, but remained consistently highest in the Great Lakes. For chlordane-related residues, mean concentrations were lower that reported previously, and trans-nonachlor continued to replace cis-chlordane as the most abundant component. Collectively, these findings suggested a lower rate of chlordane influx to the aquatic environment; however, a point source of cyclodiene insecticides to the Mississippi R. near Memphis, TN, remained evident. Residues of mirex, PCA, BHC isomers, endrin, heptachlor, and HCB were either found at relatively few (< 25%) of the stations sampled in 1986 or were characterized by relatively low concentrations. Concentrations of the herbicide Dacthal (DCPA) were also low, but incidence increased from 46% of the stations sampled in 1984 to 61% in 1986. In general, organochlorine chemical concentrations were lower in 1986 than at any time reported previously. For elemental contaminants, the geometric mean, maximum, and 85th percentile concentrations (respectively, all in microgram/g wet weight) in 1986 samples were as follows: As, 0.083, 1.53, 0.24; Cd, 0.011, 0.32, 0.04; Cu, 0.794, 11.0, 1.7; Hg, 0.087, 0.44, 0.18; Pb, 0.058, 1.90, 0.21; Se, 0.417, 3.41, 0.66; and Zn, 21.191, 94.5, 31.7. Mean concentrations of Cu increased and concentrations of As decreased relative to the 1984 collection, but these changes may reflect subtle differences in the species composition of the 1986 collection relative to other collections; concentrations of both elements differ greatly among fishes. There were no other statistically significant changes from 1984 to 1986; however, mean concentrations of As, Cd, Pb, and Zn declined from 1976, when elemental contaminants in fish were first measured in the NCBP, and 1986. In contrast, mean concentrations of Hg and Se did not change appreciably. Moreover, and in contrast to the other elements measured in 1986, concentrations of Hg and Se were high enough to constitute a threat to pisc