Plomestane is an enzyme-activated aromatase inhibitor. Plomestane have demonstrated their ability to irreversibly inhibit estrogen biosynthesis in human placental microsomes. Plomestane have a dose-dependent effect on the metabolism of androstenedione in vitro in human breast tissues. It is selective inhibitors of aromatization in human breast tissues and may provide a mechanism for controlling estrogen responsive processes. Prolonged administration of plomestane to rats bearing dimethylbenzanthracene-induced mammary tumors resulted in significant regression of hormone-responsive tumors within several days. In animal models, the effects of plomestane on both regression of existing tumors and the appearance of new tumors were reversed by co-administration of estradiol. Thus, plomestane impairs estrogen-dependent mammary tumor growth, resulting in cessation of new growth and regression of responsive tumors.