Although much attention has been focused on the synthesis of dihydrotestosterone (DHT), the inactivation and elimination of active androgens can also be key points in regulating androgen levels in tissues such as the prostate. Recent data suggest that 5alpha-reduced C19 steroids can be converted to glucuronide derivatives in the human prostate, leading to complete inactivation of these steroids. These results are supported by the recent finding of at least two steroid uridine diphosphoglucuronosyltransferase (UGT) enzymes in the prostate as well as in the human prostatic cancer LNCaP cell line. To ascertain the role of UGTs in regulating active steroid levels, we investigated the modulation of UGT levels in response to steroid treatments in LNCaP cells. Results demonstrate the down-regulation of UGT activities specific for 3-hydroxysteroids and 17-hydroxy-steroids after treatment with androgens and estrogens. Treating the cells with DHT or R1881 for 7 days inhibited UGT activity by 60%; however, 80% of the total activity was recovered after 5 days in the absence of the androgens. The inhibition of UGT activities by DHT and R1881 increases with the time of incubation and with increasing concentrations of the androgens used. The decrease in UGT enzyme activity occurred in parallel with a diminution in UGT transcript levels, as observed in Northern blot analyses. A correlation between the effect of steroids on the androgen-dependent growth response of LNCaP cells, the secretion of prostate-specific antigen, and the inhibition of UGT activities was clearly demonstrated, which implicates the androgen signaling pathway. Treating cells with Casodex, an androgen antagonist that binds the mutated androgen receptor expressed in LNCaP cells, partially blocked the androgen- and estrogen-induced decrease in UGT activity, suggesting that the regulation of UGT levels involves the androgen receptor. In addition to the formation of DHT, the inactivation of steroids by glucuronidation, which is regulated by steroids themselves, is an important mechanism controlling the level of androgens in the prostate.