Portal venous pressure measurements were made through percutaneous transhepatic portal vein catheterization in 29 cirrhotics with portal hypertension and in seven controls before and after intravenous vasopressin infusion in a dosage of 0.24 IU/min. In cirrhotic patients the decrease in portal pressure was 8.5% being not statistically significant, while in the control group the decrease was 12% (p less than 0.05). Five of six cirrhotic patients (85%) whose acute bleedings were previously controlled with i.v. vasopressin infusion had reversed portal vein flow. On the other hand, only three of ten (30%) patients whose bleeding failed to be controlled with vasopressin had reversed flow in the portal vein. The difference between these two groups is statistically significant (p less than 0.05) and may suggest that variceal bleeding is most effectively controlled with i.v. vasopressin in cirrhotics with well developed collaterals and reversed portal vein flow.