Hepatic transferrin receptors were studied in normal male rats at 1 to 59 wk after weaning, using immunohistochemical and biochemical techniques. The number of transferrin receptors measured and the intensity of the staining in situ decreased rapidly during the first 10 wk of life and more slowly thereafter. Immunohistochemistry further demonstrated changes in the topographical and (sub)cellular localization of the transferrin receptor. In the young rat livers, staining was almost exclusively present on hepatocytes in acinar zone 2 + 3 in a honeycomb to sinusoidal pattern. With aging, a panacinar heterogeneous and mainly sinusoidal staining of hepatocytes was more frequent. Kupffer cell positivity was more obvious as compared with the young rat livers. The observed changes in transferrin receptor expression may partly be explained by age-dependent alterations in DNA synthesis and proliferative potential of the liver cells. A series of rats were iron loaded with carbonyl iron up to 39 wk and "unloaded" by administration of a normal diet during 20 wk. In these animals, serial histochemical studies showed predominantly parenchymal (7 to 14 wk), mixed parenchymal and reticuloendothelial (39 wk) and almost exclusive reticuloendothelial siderosis (59 wk). In the siderotic livers transferrin receptor numbers tended to be lower than in the controls with significant differences after 14 and 39 wk. Immunohistochemistry showed decreased parenchymal but increased reticuloendothelial transferrin receptor expression with iron load. After the period of unloading, parenchymal transferrin receptors were virtually absent despite the negligible siderosis of these cells. In contrast, siderotic reticuloendothelial cells were intensely positive. These findings support down-regulation of parenchymal transferrin receptor resulting from iron storage. However, the positivity of siderotic reticuloendothelial cells and the absence of re-emergence of parenchymal receptors in conditions of minimal parenchymal and prominent reticuloendothelial siderosis need further elucidation.