The distribution of immunoreactive glucagon, substance P (SP) and vasoactive intestinal polypeptide (VIP)-like structures was investigated in the rat liver, with special reference to the hepatic vasculature by means of the indirect immunofluorescence method. Immunoreactive structures of glucagon were seen in the walls of the portal vein, hepatic artery and hepatic vein, but not in the central vein. Immunoreactive glucagon was localized in the smooth muscle cells of these blood vessels. SP and VIP-like immunoreactive (SPI and VIPI) structures were seen in the neuronal elements. In the porta hepatis, thick, compact SPI and VIPI fibers, which were dissociated from their fiber bundles, reached the tunica adventitia where they were distributed. No SPI and VIPI structures were seen in the tunica media or the tunica interna. No SPI- and VIPI-containing cell bodies could be detected in the liver. These observations suggest that these peptides may have an important role in the neural regulation of hepatic hemodynamics.