The effects of portal, hepatic arterial and femoral venous administration of prostaglandin E1 (PGE) on portal venous flow (PVF) and hepatic arterial flow HAF were examined before and after 70% hepatectomy in anesthetized dogs. In the hepatectomized condition, portal venous administration of PGE (0.5 microgram/kg/min) caused an increase in PVF without any change in systemic arterial pressure (SAP). HAF was unchanged following the injection. The portal effect of PGE on PVF was dose-dependent, and a reduction in portal venous resistance was seen. However, the same dose of PGE failed to change PVF under intact liver conditions. Hepatic arterial administration of PGE (0.5 microgram/kg/min) brought no significant change in PVF or HAF, with or without hepatectomy. Femoral venous administration of PGE (0.5 micrograms/kg/min) produced an increase in PVF concomitant with a significant decrease in SAP. HAF showed no change after the injection. A decrease in PVR was seen only in the hepatectomized condition. It is concluded that PGE is potent in increasing PVF in the hepatectomized condition, and the portal vasculature is involved as the site of action.