Abstract This study was designed to provide information on the subchronic toxicity of octachlorostyrene (OCS), a demonstrated environmental pollutant in fish from the Great Lakes of North America and the Norwegian coast in Europe. Groups of 15 male and 15 female rats were administered OCS mixed in the diet at 0.05, 0.5, 5.0, 50, or 500 ppm for 13 weeks. Increased liver weights were observed in male and female rats fed 50 ppm OCS and higher, while enlarged kidney and spleen were noted in the highest dose groups. Hepatic microsomal enzyme induction occurred at 5.0 ppm OCS and higher for the males and 50 ppm and higher for the females. The chemical produced serum biochemical changes at concentrations as low as 5.0 ppm. OCS treatment resulted in hematological disturbances starting with the 0.5-ppm dose group. Dose-dependent histological changes were observed in the thyroid, kidneys, and liver of the treated animals. OCS residues accumulated in a dose-related fashion in the liver and fat of treated animals. These results indicate that OCS produced toxic effects at low levels of exposure and accumulated in the tissues of rats.