Parumbilical (portal) vein, arterial, and central venous pressures were monitored continuously during sham and vasopressin injection (Pitressin) infusion in ten adult patients in the intact, unanesthetized state. Five of these patients at rest (group A) had portal pressures greater than, and five (group B) lower than, 30 cm H2O. Twenty clinical units of vasopressin injection infused intravenously over 20 minutes caused an average fall in portal pressures of 19.8% (group A) and 25% (group B), abrupt arterial pressure elevations with bradycardia, and no changes in central venous pressure (CVP). Side portal flow diminished an average of 21%, and variable elevations of lactate levels in portal blood were observed. Valsalva maneuvers caused 46 to 64 cm H2O increases in portal pressure, unaffected by administration of the drug. In none of the patients in group A did vasopressin succeed in lowering portal pressure below 30 cm H2O.