An in vivo technique for assessment of the relative contributions of hepatic artery and portal vein to liver perfusion has been developed in the rat. Dynamic scintigrams have been obtained following i.v. bolus injection of Tc-99m sulfur colloid. Temporal separation of the arterial and venous phases has been verified by hepatic-arterial ligation and portacaval diversion. The former procedure abolishes the early arterial phase of normal uptake. Portacaval diversion similarly eliminates the delayed venous phase. Assessment of the individual components of liver perfusion is of promise in the investigation of hepatic dysfunction.