Liver, spleen, trunk kidney, gills, and brain of rainbow trout (Oncorhynchus mykiss) were examined histologically after exposure to different concentrations of methiocarb (2.5 and 3.75 mg/L) or endosulfan (0.6 and 1.3 mu g/L) for 21 days. Histological recovery was also studied by maintaining the pesticide-exposed fish in a freshwater system for an additional 30 d. Lesions were not evident in liver, kidney, spleen, or brain of fish exposed to either concentration of methiocarb for 21 d. Lesions were observed in gills, liver, spleen, and trunk kidney (but not brain) of rainbow trout exposed to either concentration of endosulfan. There was no concentration-related effect observed on the histopathological lesions. After 30 days of recovery, fish had no histological lesions in gills, kidney, spleen, liver, or brain. Therefore all the changes observed during exposure were reversible.