Purified carrier-free 125I-Tyr insulin (125I-Tyr Ins) was injected into the vitelline vein of rat fetuses in utero after 17, 19, or 21 days of gestation. Three minutes later, a radioactivity concentration index (RCI) was calculated by dividing the specific activity of each organ by that of the feto-placental unit. The highest RCI were found in the liver (4.09, 5.97, and 6.48, respectively after 17, 19, and 21 days of gestation). Binding of 125I-Tyr Ins to the liver was inhibited by coinjection of excess unlabeled insulin. Sequential injections of 125I-Tyr Ins followed by excess unlabeled insulin demonstrated that 125I-Tyr Ins binding to the liver was but partly reversible and allowed us to calculate that the half-life of the tracer in the whole body receptor compartment was 6 min in 21-day postcoitum fetuses. After extraction and chromatographic analysis, liver radioactivity appeared composed of undamaged 125I-Tyr Ins and low mol. wt. degradation products (125I- and 125I-tyrosine). Liver maturation was characterized by an enhanced capacity to degrade 125I-Tyr Ins and to expel low mol. wt. radioactive products out of the cells. Autoradiographic studies demonstrated that 125I-Tyr Ins was bound in a saturable manner to the hepatocytes and, to a lesser extent, to the liver hematopoietic cells. Three minutes after the tracer alone, silver grains were predominantly associated with the hepatocytes′ plasma membranes. Later, the percentage of internalized grains increased. Because the percentage of liver radioactivity identified as low mol. wt. radioactive products was always smaller than that of internalized silver grains, true 125I-Tyr Ins was actually internalized in the hepatocytes. The rate of 125I-Tyr Ins translocation from the membrane into the cytoplasm increased with the degree of liver maturity. It is concluded that during the last 5 days of gestation, liver maturation concerns insulin internalization and degradation, i.e., postreceptor steps rather than receptor ontogenesis.