The binding capacity for estradiol in the cytosol and nucleus of the rat posterior hypophysis was demonstrated in vitro. The maximal 3H-estradiol binding took place at 37°C in both fractions; but when time experiments were performed, the most binding occurred at 30 min in the cytosol and at 90 min in the nucleus. Administering exogenous estradiol to spayed rats shifted the site of maximal binding from the posterior hypophysis’ cytosol to the nucleus. Both fractions showed a low capacity for 3H-estradiol binding; we studied the presence of a macromolecular component(s) that bind(s) estradiol with high affinity in the cytosol. The number of binding sites in the whole posterior hypophysis was determined in spayed and estradiol-treated rats. In the latter group, the protein content of the posterior hypophysis increas