We studied the effects of androgens on basal and FSH-stimulated aromatase activity in Sertoli cell-enriched monolayers. Daily exposure to androgens from the first day of culture results in a time- and dose-dependent decrease in inducible aromatase activity. The inhibition is observed whether FSH, L-isoproterenol or (Bu)2cAMP is used as inducer of the aromatase activity. Basal activity is not affected by preincubation with androgens and the inhibitory effect is not observed after short-term exposure (24 h) to these hormones. The ability of different androgens to decrease inducible aromatase activity does not depend on their ability to serve as a substrate for the aromatase but parallels their ability to stimulate the production of androgen-binding protein. Moreover, the effect of testosterone is neutralized by the antiandrogen cyproterone acetate, suggesting that it is mediated by the androgen receptor. These data suggest that testicular androgens may be responsible for the decrease in FSH-inducible aromatase activity observed in intact rats between days 10 and 30 of life. Similarly, the removal of these androgens during the preparation of Sertoli cell cultures may explain the spontaneous increase in inducible aromatase activity observed when these cultures are maintained in the absence of androgens.