Marked sex differences in estradiol-binding capacity ratio of components with Stokes radii (a) 7.0 and 2.5 nm in the liver cytosol of mature intact rats were found to be diminished, but not fully eliminated by gonadectomy. Prolonged administration of estradiol (50 microgram/animal for 8 days) to gonadectomized rats led to inhibition of the estradiol-binding activity of all the components in the liver cytosol of male and female animals. Administration of testosterone-propionate (2 mg/animal for 8 days) to gonadectomized animals stimulated selectivity the peculair estrogen-binding protein with a 2.5 nm not only in male, in which this protein was normally found, but also in female rats. It is supposed that sex differences in the system of estradiol-binding proteins in rat liver cytosol are determined by sex differentiation of the system at the early stages of ontogenesis, on the one hand, and by the active regulating influence of estrogens and androgens at the late stages of ontogenesis, on the other hand.