We determined the roles of liver and splanchnic vascular bed in anaphylactic hypotension in anesthetized rats, and the effects of anaphylaxis on hepatic vascular resistances and liver weight in isolated perfused rat livers. In anesthetized rats sensitized with ovalbumin (1 mg), an intravenous injection of 0.6 mg ovalbumin caused not only a decrease in systemic arterial pressure from 120±9 to 43±10 mmHg but also an increase in portal venous pressure (Ppv) which persisted for 20 min after the antigen injection (the portal hypertension phase). The elimination of the splanchnic vascular beds, by the occlusions of the celiac and mesenteric arteries, combined with total hepatectomy attenuated anaphylactic hypotension during the portal hypertension phase. For the isolated perfused rat liver experiment, the livers derived from sensitized rats were hemoperfused via the portal vein at a constant flow. Using the double occlusion technique to estimate the hepatic sinusoidal pressure, pre- (Rpre) and post-sinusoidal (Rpost) resistances were calculated. An injection of antigen (0.015 mg) caused venoconstriction characterized by an almost selective increase in Rpre rather than Rpost, and liver weight loss. Taken together, these results suggest that liver and splanchnic vascular beds are involved in anaphylactic hypotension presumably due to anaphylactic presinusoidal contraction-induced portal hypertension, which induced splanchnic congestion resulting in a decrease in circulating blood volume and thus systemic arterial hypotension.