The effects of prolonged oral administration of the alpha-, beta-, and gamma-isomers of benzene hexachloride (BHC) on rat liver were examined histologically. Well-differentiated hepatocellular carcinomas were observed in the liver of rats fed a basal diet containing 1,500 or 1,000 parts per million (ppm) of alpha-BHC for 72 weeks. Many typical nodular hyperplasias developed in all groups given 1,500 or 1,000 ppm alpha-BHC. In non-neoplastic areas, slight oval cell infiltration and bile duct proliferation were seen. No neoplastic changes or other abnormal findings, such as oval cell infiltration, fatty changes, fibrosis, or bile duct proliferation of the liver, were observed in groups receiving 500 ppm alpha-, beta-, gamma-, or gamma-BHC.