Endrin toxicity may be due to an oxidative stress associated with increased lipid peroxidation, decreased glutathione content, and inhibition of glutathione peroxidase activity. Extensive interspecies variability exists in sensitivity towards endrin. Therefore, histopathological changes and lipid peroxidation in the livers and kidneys of rats, mice, hamsters, and guinea pigs were examined 24 hr after the administration of 4 mg endrin/kg body weight orally in corn oil. Degeneration and necrotic changes with inflammatory cell infiltration were observed in livers and kidneys, and interspecies variability occurred. Fatty changes in the form of hepatic foam cells with cytoplasmic vacuolation were present. Lipofuscin pigments, associated with lipid peroxidation, were observed in hepatocytes and Kupffer cells. These histopathological conditions were prevented in rats which had been pretreated with butylated hydroxyanisole, vitamins E and C, or cysteine, antioxidants and free radical scavengers which have previously been shown to inhibit lipid peroxidation. The extent of endrin-induced lipid peroxidation correlated well with the degree of histopathological changes. Thus, histological changes consistent with the induction of an oxidative stress were observed following the administration of endrin to various animal species.