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CGS 16949A inhibited the conversion of [4-14C]androstenedione (A) to [4-14C]estrone by human placental microsomes in a competitive manner (Ki = 1.6 nM). Aminoglutethimide, also a competitive inhibitor, had a Ki = 0.7 microM in this assay system. The Km for the aromatization of A was 0.11 microM. Using ovarian microsomes from immature rats primed with pregnant mare's serum gonadotrophin and using [4-14C]testosterone conversion to [4-14C]estradiol as a measure of aromatase activity, the Km was 42 nM. At a substrate concentration 3-fold the Km, CGS 16949A was 180 times more potent as an inhibitor than aminoglutethimide, exhibiting half-maximal inhibition at 1.7 nM as compared to 0.3 microM. In vivo CGS 16949A lowered ovarian estrogen synthesis by gonadotropin-primed, androstenedione treated, immature rats by 90% at a dose of 260 micrograms/kg (PO). A dose of 100 mg/kg of aminoglutethimide was needed to produce this same effect. CGS 16949A at a dose of 4 mg/kg (PO) induced uterine atrophy (aromatase inhibition) without inducing adrenal hypertrophy - indicating a lack of inhibition of corticosterone secretion, while aminoglutethimide at 40 mg/kg (PO) induced adrenal hypertrophy without inducing uterine atrophy. CGS 16949A was neither androgenic nor estrogenic in rats using standard bioassays. The data suggest that CGS 16949A may serve as a potent and selective agent for modulating estrogen-dependent functions.