PIP: It has been suggested that tamoxifen possesses estrogenic properties which may induce regression of mammary tumors in humans. However, it has been shown that tamoxifen inhibits the estradiol-stimulated changes in immature, ovariectomized, and mature rats. Whereas estradiol stimulates cell division, tamoxifen produces endometrial hypertrophy, even though it has been shown to bind to cytoplasmic estrogen receptors. In vitro experiments with human breast cancer cells have shown that tamoxifen has effects other than that of a simple estrogen antagonist. It is concluded that in addition to its antiestrogneic properties, it may act as a partial or a typical estrogen agonist, which may account for its antitumor activity in postmenopausal patients with breast cancer.