Estrogens have been suggested as modulators of the conversion of 25-hydroxyvitamin D3 to dihydroxylated compounds in the kidney. In order to further explore this hypothesis the estrogen-binding components in the kidney were studied in adult and immature rats. The basal receptor levels in adult animals were 9.6 fmol/mg protein (female) and 21.9 (male). The receptor-ligand complex had a Kd of 0.7 nM. Furthermore, the kidney receptor displayed similar characteristics as those of the cytosol liver estrogen receptor in terms of sedimentation properties on sucrose gradients, isoelectric focusing and ligand binding specificity. The ontogeny of cytosol high affinity estrogen binding sites was elucidated in female and male animals. Detectable levels of receptors (5 fmol/mg protein) were found during the first postnatal week in both sexes. During days 22-25 receptors reached maximum concentrations at about 30 fmol/mg protein. In the male this level then remained relatively constant throughout the time of study (60 days), whereas in the female the concentration decreased gradually over a period of 12-15 days to a basal level of 10 fmol/mg protein. A temporal study on the short- and longterm effects of ovariectomy on the concentration of estrogen binding sites in the kidney cytosol was also carried out. Shortly after gonadectomy (2-12 h) no effect was detected. During 20-48 h after the operation a 75% increase in the receptor level was seen. The results indicate a multihormonal control of the estrogen binding protein in the kidney similar to that seen in the liver. Furthermore, the data suggest that estradiol down-regulate its own receptor. The results are discussed in relation to present concepts on the actions of estrogens and the metabolism of vitamin D3.