Synthetic and natural antiuterotrophic and uterotrophic compounds were tested invitro in their ability to bind to the uterine cytosol receptor for estradiol (Rc) and to favor the formation of the nuclear receptor. These compounds were compared to estradiol in their capacity to decrease the number of specifically bound estradiol in cytosol and increase those in nuclear extracts when incubated with uteri invitro. For each compound, a ‘nuclear transfer activity’ was defined and compared on the one hand to its binding affinity for Rc and on the other hand to its invivo uterotrophic activity. For estrogens devoided of antiestrogenic activity, estradiol and estrone, the nuclear transfer activity was parallel to the affinity for Re and the biological activity. The antiestrogen nafoxidine (U11,100) when added to the cytosol was found to be a reversible and competitive inhibitor of estradiol for Re with an affinity about 30-fold less than estradiol (Ki 7 nM). A complete inhibition could be obtained both before and after the dissociation of Rc by KCl. However, these binding properties of nafoxidine were not in agreement with its partial nuclear transfer activity and its partial uterotrophic activity. The androgens do not bind to Rc; however, they were shown to activate the transfer of the estrogen Rc to the nucleus whereas progesterone did not. These results are discussed in relation to the mechanism of action of uterotrophic and antiuterotrophic compounds.