Abstract Immature female rats were primed with a non-ovulating dose of pregnant mare serum gonadotropin (PMS), at 24 days of age, and later treated by an antisteroidogenic drug — either 17β-acetamidoandrost-4-ene-3-one (AAA), a competitive inhibitor of the steroid 17,20-lyase, or aminoglutethimide phosphate (AGP), an inhibitor of hydroxylase-associated cytochrome P-450 action. Both drugs initiated a fall in the ovarian androgen content, concomitant with decreased ovarian aromatase activity, manifested in vivo and in vitro. Testosterone therapy, administered to the antisteroidogenic drugs-treated rats, elevated the ovarian estradiol-17β production only in the AAA-treated ones. This elevation, detectable both in vivo and in vitro, was to values indistinguishable from controls. No such accomplishment was observed in the AGP-treated rats. Analysis of the AGP-treated rats' ovaries, 9 h after the administration of the drug, with and without a testosterone supplement, revealed residual amounts of the drug sufficient to inhibit the aromatase, both in vivo and in vitro. The data support the theory suggesting that reduced androgen levels, due to the deactivation of the 17,20-lyase, result in cessation of estrogen secretion from the ovary.