We studied 58 patients undergoing orthotopic liver transplantation, aged 42 +/- 10 years (mean +/- SD), and weighing 65 +/- 14 kg. Anesthesia was maintained with fentanyl, midazolam, and vecuronium. Serum bicarbonate, serum potassium, serum ionized calcium and pH did not change significantly throughout the study. Usual hemodynamic parameters were recorded. Hemodynamic tolerance was assessed by a trial of clamping of the inferior vena cava, above and below the liver and the portal vein; patients were allocated to two groups: the group without venovenous bypass (NBP, n = 29) consisted of patients whose MAP did not decrease by more than 30% and/or cardiac output did not decrease by more than 50%; the group with venovenous bypass (BP, n = 29) consisted of patients whose MAP decreased by more than 30% and/or cardiac output decreased by more than 50% or required venovenous bypass for easier surgical dissection. After clamping of the vena cava and the portal vein, the cardiac index (CI) and mean pulmonary arterial pressure (MPAP) decreased significantly, whereas systemic vascular resistances (SVR) increased. After unclamping the inferior vena cava suprahepatically and infrahepatically, no hemodynamic change was observed. After unclamping the portal vein, MAP decreased, despite the increase in the CI, because of an significant decrease in SVR; in addition MPAP increased despite the decrease in pulmonary vascular resistances. The decrease in MAP of more than 30% during at least 1 min occurred in 6 patients (20%) in the NBP group and in 6 patients (20%) in the BP group. We concluded that the occurrence of the syndrome of cardiovascular collapse following liver reperfusion was similar whether venovenous bypass was used or not.