Accumulation of different estrogen-receptor (ER) complexes depending on the corresponding equilibrium and kinetic parameters of the ER interaction in guinea pig uterus cytosol was studied. High and significant correlation (r = 0.99, p less than 0.01) was found between the complex accumulation in nuclei and the values of the constant association rates. The data obtained suggest that intracellular ER interaction (at limited periods) was not an equilibrated but a quasistationary process. Activation of the ER complexes and their translocation into nuclei occurred unspecifically and independently on the properties of the ligand moiety. The translocation does not depend on presence of other competitive ligands in a reaction mixture or in cytoplasm of a target cell.