The hepatic arterial and hepatic portal venous vascular beds of the chloralose-urethane anesthetized dog were perfused simultaneously in situ. Vasopressin (10 mU = 1 unit) was injected in graded increasing doses into the hepatic artery and into the portal vein. Both intra-arterial and intraportal vasopressin elicited both hepatic arterial vasoconstriction and hepatic venous dilation; the delay in onset of both hepatic vascular effects was significantly shorter than that for any succeeding systemic effects (a rise in systemic arterial pressure and fall in heart rate), showing that they were not attributable to recirculation or to arterial baroreceptor reflexes. Injections of vasopressin into the inferior vena cava at the level of the hepatic veins consistently produced smaller hepatic vascular effects than either intra-arterial or intraportal injections of the same doses. The results are discussed in the context of the therapeutic role of vasopressin in controlling gastrointestinal bleeding and portal hypertension.