Heme oxygenase (HO), the heme-degrading enzyme, plays an important role in heme catabolism. Among three isozymes, HO-1 is an inducible form expressed mainly in macrophages. In rat ontogeny, HO-1 immunoreactivity was detected in mononuclear cells in the yolk sac at 10 days of gestation. HO-1-expressing cells were then detected in the fetal liver and their numbers increased during the gestational period. The numbers of HO-1-positive cells and HO-1 mRNA levels in the liver peaked at 18 days of gestation. Most of the macrophages expressed both HO-1 and a macrophage scavenger receptor. Macrophages in the fetal liver showed marked hemophagocytosis. Macrophages in the lung, spleen, bone marrow, and other tissues also expressed HO-1. HO-1 immunoreactivity was also observed in syncytial cells of the chorionic villi, the endodermal layer of the yolk sac, and renal tubules of the fetus. Intestinal mucosal epithelial cells expressed HO-1 after birth. These findings imply that HO-1 is crucial for macrophages in heme catabolism from an early stage of ontogeny. HO-1 expression in non-macrophagic cells may be required for other purposes such as protection from oxidative stress and various stimuli.