Studies are reported using unlabelled and radiolabelled antiestrogens which are aimed at elucidating the nature of interaction of antiestrogens with uterine and mammary tissue. Antiestrogens and long-acting estrogens move estrogen receptor sites to the nucleus and maintain elevated nuclear receptor levels for a prolonged period. The antagonistic action of antiestrogens in the uterus, however, appears to derive from their ability to effect a marked perturbation in the subcellular distribution of receptor whereby very little (ca. 10%) is cytoplasmic and further estrogen receptor accumulation is blocked. In DMBA-induced rat mammary tumors, the nonphototoxic antiestrogen, U-23, 469, effectively antagonizes the development and growth of tumors and elicits regression of 90% of established tumors with a time course similar to that seen after ovariectomy. Again, during antiestrogen treatment, the levels of cytoplasmic estrogen receptor are depressed with over 90% of total receptor being found in the nucleus. Two radiolabelled antiestrogens of high specific activity and purity have been prepared by us and their interaction with nuclear and cytoplasmic estrogen receptors is reported. Both 3H-CI-628 and 3H-U-23, 469 are metabolized in vivo to more polar forms which are found associated with the nuclear estrogen receptor, and which may be the true agents active in vivo.