To date, few studies have been reported on any aspect of DNA damage in organs of intact animals. We and others demonstrated induction of high incidences of dysplasia, a precancerous proliferative lesion, and adenocarcinoma selectively in the dorsolateral prostates (DLPs) of Noble rats exposed to a long-term combined testosterone and estradiol-17 beta treatment (T + E2-treatment). We here report induction of a significant increase in DNA strand breakage and accumulation of lipid peroxidation fluorescent products in the DLPs, but not in the ventral prostates (VPs), of rats treated with T + E2 for 16 weeks. These results indicate that this dual hormone treatment has free radical-based DNA damaging effect on rat DLP and therefore may have tumor-initiating action. Since a similar effect was not observed in regenerating DLPs of castrated rats following testosterone replacement, we conclude that the genotoxic effect of the dual hormone treatment is a direct result of hormone action and not secondarily due to enhancement of cell proliferation.