Portal venous flow (PVF) was examined after portal injection of ursodesoxycholic acid (URSO) in rats that were partially hepatectomized by either 40% or 66%. URSO (10 mg/kg per minute) was injected into the portal vein and was thereafter observed to increase PVF concomitantly with a fall in portal vein and was thereafter observed to increase PVF concomitantly with a fall in portal venous pressure (PVP) in control animals. The increase in PVF in response to URSO was dose-dependent. In hepatectomized rats, the PVF response was augmented when the same dose of URSO was portally injected, and the magnitude of response was enhanced in proportion to the volume of liver resected. These results suggest that URSO increases PVF through vasodilation of the portal vessels, and therefore URSO is considered to increase PVF potently in a partially hepatectomized condition.