A comparative in vitro study of interaction of estradiol (E2) complexes with different forms of cytosol estrogen receptors (ER) from rat liver, kidney and uterus, and cell nuclei was carried out. It was shown that complexes of E2 with ER of whole liver and kidney cytosol of female rats as well as complexes of E2 with forms I and II of liver ER separated from one another and partially purified can be translocated into cell nuclei, similar to the estrogen-receptor complexes (ERC) of uterine cytosol. ERC, which contain an unusual estrogen-binding protein of male rat liver cytosol, do not bind to the nuclei in vitro. The translocation into the nuclei of high molecular weight ERC from liver and uterus has some common features, e. g. the necessity of activation by various factors, similar kinetics, absence of marked organ specificity, non-saturation of nuclear binding sites for ERC, etc. In terms of these criteria (with the exception of kinetic patterns) the translocation of forms I and II of female rat liver ER appears to be identical. The kinetic patterns and the level of translocation into the nuclei and the binding to DNA-cellulose of forms I and II of liver ER at low ionic strength reveal some differences, i. e. form II is characterized by faster kinetics, but by a lower level of interaction with the nuclei and DNA-cellulose. These differences disappear as the ionic strength of solution rises. The fast kinetics of ERC translocation into the nuclei at low ionic strength is also typical for kidney cytosol ER which are similar to those of form II of liver ER. The data obtained suggest that various forms of ER are involved in initiation of a broad spectrum of hormonal effects of Er both in the cells of different tissues and in homologous cells of the same tissue.