The aim of the present work was to study the effects of estradiol (E2) on cell proliferation in well-differentiated mammary MXT tumors borne either by intact (INT), or by castrated (OVX), or by hypophysectomized (HX), or by castrated and hypophysectomized mice) OV-X-HX). Ovariectomy and hypophysectomy were performed, respectively, 6 days and 5 days before the sacrifice of animals bearing tumors during a 28 day period. Cell proliferation was measured by using an in vivo nuclear labeling with tritiated thymidine (3H-TdR) followed by autoradiography. Uteri were used as controls for the methodology. Our data demonstrate that E2 exerts a significant and transient stimulatory effect on cell proliferation in the uterine luminal and glandular epithelia of OVX or OVX-HX adult mice. This mitogenic enhancement is significantly higher in OVX-HX mice than in the other experimental groups. In MXT tumors, a significant increase of cell proliferation is induced by E2 in INT or OVX mice; however, no enhancement occurs after HX performed either in intact or in castrated mice. In the latter situation (HX), a dramatic increase of basal cell proliferation rates is consistently observed in tumors, as compared to those levels observed in INT or in OVX animals. The paradoxical increase in basal proliferation rates of MXT mammary tumors in HX animals was not suppressed by administration of ovine prolactin. This suggests that other hypothalamo-hypophysial factors might be endowed with direct inhibitory influence on growth of the MXT tumors.