Haemodynamic parameters, including cardiac output, blood volume, and blood pressures in the systemic and pulmonary circulations, were followed in eight patients undergoing porto-caval shunt operations because of portal hypertension. Measurements were performed in the awake patients before premedication, during operation immediately before clamping of the portal vein, and immediately after opening of the porto-caval shunt. Further measurements were performed 2-4 h, and 22-26 h postoperatively. Preoperatively, cardiac index and blood volume were increased, whereas the blood pressures in the systemic and pulmonary circulations were within normal accepted limits. After opening the porto-caval shunt, increases in pulmonary arterial mean pressure of between 35 and 80% and in pulmonary capillary wedge pressure of between 40 and 200% were observed in the four patients in whom the pressure gradient between the caval vein and the portal vein was normalized, i.e. where a pressure gradient after opening the porto-caval shunt was measured between 0 and 0.67 kPa. Otherwise, the haemodynamic changes were only partly related to changes in the pressure gradient. The courses of the postoperative haemodynamic changes did not seem to be different from the courses in other categories of patients. The most important haemodynamic changes in patients undergoing porto-caval shunt operations seem to take place immediately after opening the shunt. To minimize the extent of changes in the pulmonary circulation, the shunt should be opened gradually.