Research efforts over the past several years have focused on the synthesis of competitive and irreversible aromatase inhibitors and examination of these inhibitors in microsomal preparations, in cell culture, and in vivo. Several 7 alpha-substituted androstenediones have demonstrated high affinity for placental aromatase, with apparent Ki's ranging from 1 to 30 nM. Inactivation of aromatase occurred following incubation with alkylating and enzyme-activated irreversible inhibitors. 7 alpha-(4'-Amino)phenylthio-4-androstene-3,17-dione (7 alpha-APTA) exhibits potent inhibitory activity of aromatase in the MCF-7 human mammary carcinoma cell line with an ED50 of approximately 25 nM. The inhibitor did not bind to the estrogen receptor of the cells in vitro nor induce levels of progesterone receptors in intact cells. In vivo studies of 7 alpha-APTA in the DMBA-induced rat mammary carcinoma model resulted in 80% of the tumors responding completely or partially at doses of 25 and 50 mg/kg body wt/day. Thus, these 7 alpha-substituted steroidal aromatase inhibitors are effective medicinal agents and may be useful for the treatment of estrogen-dependent breast cancer.