In this investigation, the activities of five enzymes (catalase, superoxide dismutase, glutathione peroxidase, glutathione reductase, glucose-6-dehydrogenase) involved in oxygen radical (OR)-detoxification and antioxidative defense were measured in conjunction with an index of lipid peroxidation in major prostatic lobes of young (2 month-old) and old (9 month-old) rats. Selective decreases in catalase and glutathione peroxidase activities were observed in the ventral prostate lobe (VP), while a general decline in the activities of all five enzymes studied was noted in the dorsolateral prostate lobes (DLPs) of senescent rats. The extent of lipid peroxidation, measured as concentrations of thiobarbituric acid-reactive products, was greatly elevated in both VP and DLP of old rats. These results demonstrate for the first time that reduction in antioxidant capacity and elevation in peroxidative damage occur in rat prostate during aging. aging. We postulate that these prooxidative alterations promote diseases in the aging prostate.