The effects of 10 and 15 cm H2O positive end-expiratory pressure (PEEP) on portal vein flow (PVF), superior mesenteric artery flow (SMAF), and cardiac output (CO) were studied. Portal vein pressure (PVP) and the intraluminal pressure gradient between portal vein and intrathoracic inferior vena cava (PVP-CVP) were also measured. PEEP caused a fall in CO and SMAF, and the fall in each was proportionately the same. PVF also fell and a linear correlation between PVF and SMAF during PEEP was observed. Although PVP increased relative to atmosphere, no increase in transmural PVP and no change in PVP-CVP gradient was observed with PEEP. These data suggest that the major effect of PEEP on portal hemodynamics is a reduction in portal flow secondary to the reduction of arterial inflow into the gut. These hemodynamic changes do not appear to be the result of an increase in resistance to protal flow at the liver but rather the result of PEEP reducing cardiac output.