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We showed previously that exogenously administered testosterone caused age- and lobe-specific overgrowth of the prostate in Brown Norway rats. A common feature observed in testosterone-treated animals was cell hypertrophy in each of the ventral, dorsal, and lateral lobes of both young (6 mo old) and old (24 mo old) rats. By contrast, hyperplasia was seen only in the dorsal and lateral lobes of old rats treated with testosterone. These observations prompted us to examine whether age- and lobe-specific overgrowth might also occur in untreated rats as a consequence of the endogenous hormonal milieu. To this end, blood and prostates were collected from a large number (25–30 rats per group) of 4- to 6-mo-old (young) and 21- to 24-mo-old (old) Brown Norway rats. Both serum testosterone (−45%) and estradiol (−22%) concentrations decreased significantly with age, but the greater magnitude of the …