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Rainbow trout ( Salmo gairdneri ) fingerlings were exposed to sodium hypochlorite (NaClO) in semistatic LC 50 tests for 48 h in dechlorinated Helsinki tap water and water from Lake Vatia (21 km downstream from a kraft pulp mill). Fish were also exposed for 17 days to Lake Vatia water after which an acute exposure to sodium hypochlorite was made, for which physiological samples were analyzed. The 48 h LC 50 of bleached kraft pulp mill effluent was 35%. The 17 days' caging period in the recipient water 21 km downstream from the mill caused disturbances in osmotic regulation, depletion of liver glycogen and inhibition of liver UDP-glucuronosyltransferase. The gills of the exposed fish were microscopically examined. The epithelium of the secondary lamellae in the NaClO exposed gills was thicker than in the controls (48 h LC 50 0.35 mg Cl 2 /l) and the lamellae were curled with constricted blood spaces. The gill epithelium of fish exposed to NaClO in water from Lake Vatia was swollen and vacuolized (48 h LC 50 0.09 mg Cl 2 /l). The gills of fish exposed to 20% pulp mill effluent as well as the gills of fish acclimatized for 17 days in Lake Vatia water were quite normal in structure. However, after hypochlorite exposure the gills of the pre-acclimatized fish were almost totally destroyed (48 h LC 50 0.07 mg Cl 2 /l).

The toxicity of pulp and paper mill effluents was estimated by exposing rainbow trout to the effluents in situ during four seasons. The experiments were carried out using one control group and three test groups at different distances from the discharge. After a 10-day exposure mainly hematological parameters were determined from the caged fish. The caging site affected most of the 15 parameters analyzed (blood Hct, MCHC, erythrocyte, and leucocyte counts; plasma gluc, prot, urea, Ca2+, Mg2+). Some changes were detected even at the farthest caging site, 8 km from the discharge. The time of year strongly influenced the results. Seasonal variation exceeded the variation caused by caging site. In addition, the season influenced the responses of rainbow trout to the effluent in many parameters. None of the responses detected were observed in all the experiments. The only significant changes in the median values of the parameters from the caging experiments were a decrease in the plasma Mg2+ and an increase in the plasma K+ concentration.