The localisation and ontogenic expression of heart fatty acid binding protein (H-FABP) were immunochemically examined in the entire body of the rat. In muscles, H-FABP immunoreactivity was observed not only in cardiac muscle cells but also in red skeletal muscles, where gold particles indicating the immunoreaction sites are distributed evenly in the sarcoplasm except for the interior of membranous organelles. In the urogenital system, the immunoreactivity was preferentially demonstrated in the distal tubular epithelial cells in the kidney, and in the superficial epithelial cells in the urinary bladder. The H-FABP immunoreactivity was further found in cells characterised by numerous lipid droplets, such as interstitial cells of the theca interna in the ovary and those of Leydig in the testis, and cell Element III in the placenta. In the digestive system, parietal cells in the stomach and pancreatic beta-cells were immunoreactive to H-FABP. In addition, capillary endothelial cells were immunostained in the cardiac and red skeletal muscles, the exocrine pancreas, digestive tract and thymus. From the rather wide distribution of H-FABP, it is suggested that H-FABP plays some fundamental roles in the active metabolism of fatty acids in the body. The present findings also indicate that H-FABP is a useful marker for the morphological study of interstitial cells in the ovary and testis and parietal cells in the stomach.