Although structural changes are most important to determine vascular resistance in portal hypertension, vasoactive mediators also contribute to its regulation. Hepatic stellate cells (HSCs) are assumed to play a role in modulating intrahepatic vascular resistance based on their residence in the space of Disse and capacity to contract. Because sphingosine 1-phosphate (S1P) has been shown to stimulate HSC contractility, we wondered if S1P could regulate portal pressure. S1P at 0.5–5μM increased portal pressure in isolated rat perfused liver. This effect was abrogated in the presence of a binding antagonist for S1P₂, JTE-013. Perfusion of isolated rat liver with 5μM S1P increased Rho activity in the liver, and co-perfusion with JTE-013 cancelled S1P-induced Rho activation. Because S1P is present in human plasma at approximately 0.2μM, S1P might readily regulate portal vascular tone in physiological and pathological status. The antagonist for S1P₂ merits consideration for treatment of portal hypertension.