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The studies were done on 93 Wistar rats of both sexes. Binding capacity and affinity for 3H-estradiol were determined biochemically in the cytosol fraction of renal homogenates. Free cytoplasmic estrogen binding sites were demonstrated in a portion of the females (Kd 5.2-17.8 x 10(-10) mol/l): in 33.3% of normal animals, in 54.6% 3-4 days after ovariectomy, and in 25% 17-18 days after ovariectomy. The investigation of females which received estradiol or testosterone replacement after castration, as well as of uncastrated or castrated males, led to negative results. Thaw-mounted autoradiography was used to study the retention of radioactivity from 3H-estradiol or 3H-testosterone in normal and castrated animals and in animals which received corresponding hormone replacement after castration. In all animals, specific silver granules were found in the cytoplasm and nucleus of proximal tubules cells from the S1 and S2 segments of juxtamedullary nephrons, particularly in the females. Only a few silver granules were observed in the S segments; some cells in males were marked by a heavy nuclear accumulation of silver granules following the injection of 3H-testosterone. In the distal tubules, only isolated silver granules were found in some cells. Somewhat more radioactivity was found in the cytoplasm of the cells of the collecting ducts in the inner medullary zone.