Nuclear estrogen receptor concentrations in rat liver were determined by exchange assay. The concentration of estrogen receptors in nuclei from vehicle-treated male and female rats show age-dependent, but not sex-dependent variations in the course of life. Levels are highest during the perinatal period (approximately 1500 binding sites/nucleus), whereafter they decrease towards the onset of puberty (approximately 300 binding sites/nucleus) before rising again to reach the postpuberal maximum (approximately 800 binding sites/nucleus). Pregnancy further raised receptor concentrations in the last week of gestation when they reach approximately 1200 binding sites/nucleus. Studies with ethynylestradiol-treated rats demonstrated that virtually no translocation can be detected before the onset of puberty; thereafter the number of translocated receptors increases dramatically reaching a maximum (9000 binding sites/nucleus) between day 80 and 87 of life. The extractability of the nuclear receptors with 0.4 M KCl varies during the course of life. Extractability is very high (approximately 90%) up to about day 12 of life, but then decreases markedly (to approximately 70% in vehicle-treated and to approximately 50% in ethynylestradiol-treated rats) before the onset of puberty.