The effects of intravenous, intra-arterial and intraportal administration of glucagon on hepatic artery and portal vein flows were studied in anaesthetized dogs. Intravenous administration of glucagon (10 μg/kg) increased hepatic arterial and portal venous flows by 155% and 176% of the control value respectively. Intraarterial injection of glucagon increased hepatic artery flow and decreased its resistance. The hepatic arterial effect of glucagon was dose-dependent and was not abolished after β-adrenoceptor blockade by propranolol 1 mg/kg i.v., or after denervation. Intraportal injection of glucagon had no direct immediate (<15 sec) effect on portal venous flow or pressure. This was followed by an increase in portal vein flow and pressure reaching maximal response within 3–5 min. This late effect was most likely the result of the mesenteric vasodilator action of glucagon. It is concluded that glucagon posses a direct dilator action on the hepatic artery, and indirectly influences the portal venous circulation. Glucagon may have a physiological role in the hepatic vascular bed.