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 decrement in testosterone relative to estradiol led to a reduction in the serum testosterone:estradiol ratio. Paradoxically, although the prostate is androgen dependent, the wet weight, protein, and DNA contents increased significantly with age in the dorsal and lateral lobes of old rats despite the decrease in testosterone level. Histologic examination revealed that the increased weights and DNA contents of the dorsal and lateral lobes in old rats coincided with an increased number of epithelial cells in the distal and intermediate segments of these lobes, indicative of hyperplasia but independent of change in cell size. Taken together, these results show a spontaneous age-related overgrowth of cells in the dorsal and lateral prostatic lobes of old Brown Norway rats despite diminished serum testosterone concentrations. The aging Brown Norway rat, therefore, may be a useful model for studies of some aspects of the pathogenesis underlying spontaneous age-related prostatic hyperplasia.