The inhibitory activity of aminoglutethimide, 4-hydroxy-androstenedione and 7α-(4'-amino) phenylthioandrostenedione on human placental and mammary tumor aromatase activity was examined. [14C]-Androstenedione was incubated with placental microsomes and mammary tumor homogenates in the presence of an NADPH generating system with or without inhibitor. All three inhibitors were equally effective in inhibiting microsomal placental aromatase at various inhibitor concentrations. Inhibitions of mammary tumor aromatase using 12 μM inhibitor concentrations were essentially similar to results on placental aromatase inhibition and ranged from 81-97% inhibition. These findings are discussed in regard to the potential clinical use of aromatase inhibitors in the treatment of estrogen dependent metastatic breast carcinoma.