The effect of surgical castration of adult male Sprague-Dawley rats on peripheral and central benzodiazepine (BZ) receptors was studied. Following removal of the testes, a significant decrease in the density of peripheral BZ receptors (PBR) was observed in Cowper's glands (71%; P less than 0.005) and the adrenal (31%; P less than 0.01), but not in the heart. Administration of testosterone acetate (TA) prevented castration-induced PBR depletion. Orchiectomy per se, as well as TA administration to castrated rats, had no effect on central or peripheral BZ receptors in whole brain without the cerebellum. These results indicate the regulatory role of testosterone in the regulation of PBR in Cowper's glands and adrenal.