To provide insight into the factors that control androgen receptor levels in rat penis, we assessed 5 alpha-[3H]-dihydrotestosterone binding in low-salt [10 mM tris(hydroxymethyl)aminomethane (Tris), 10 mM Na2M0O4] and high-salt (10 mM Tris, 10 mM Na2M0O4, 0.5 M KCl) extracts of rat penis using sucrose density gradients. Total receptor content decreased from approximately 729 +/- 114 fmol/g tissue at 3 wk of age to less than 50 fmol/g tissue at 10 wk of age. Castration of 3-wk-old rats prevented penile growth and the age-related decline in penile androgen receptor. Treatment of 3-wk-old castrated rats with 5 alpha-dihydrotestosterone caused an acceleration in the decline in receptor levels compared with intact animals. Castration of 10-wk-old rats (after androgen receptor levels had decreased) did not result in an increase in the amount of total androgen receptor by 16 wk of age. To determine the specificity of the androgen-mediated decline in receptor levels, the amounts of prostate androgen receptor were compared with those of the penis at different ages. When expressed as femtomoles per organ, the total androgen receptor level in the prostate increased fourfold from 3 to 10 wk of age, whereas the total androgen receptor in the penis declined approximately threefold. We conclude that the downregulation of the penile androgen receptor content that occurs in the rat between 3 and 10 wk of age is androgen mediated, does not occur in all androgen target tissues, and is prevented but not reversed by castration.