The haemodynamic effects of portal ligation combined with mesocaval H-grafting were evaluated experimentally as follows: acute portal hypertension was induced in 20 mongrel dogs by ligation of the portal vein, in 15 of these a mesocaval shunt was constructed using an autogenous venous segment, the rest served as controls and were left without shunting. All animals with portal occlusion only died within 80 minutes, whereas nine dogs in the H-graft groups survived. The one-month patency rate was 78 per cent. Slight portal hypertension was observed only in the early post-shunt period, one month later the pressure was normal. By this time the flow through the shunt was significantly reduced and numerous but small portosystemic collaterals were visualized radiographically. Varices formation was not observed. The Xe-133 wash-out of the liver and liver pO2 were normal in the early post-shunt period. These findings were in marked contrast to those in the control group where both variables showed rapid deterioration. The pre-shunt spleen flow proved to be of prognostic significance: poor flow was associated with bad prognosis. After one month the liver flow was decreased whereas the spleen flow was increased, perhaps indicating increased splenocollateral flow. Portal ligation combined with mesocaval H-grafting thus seemed to induce slight and temporary portal hypertension in this model. This appeared to be caused by relatively narrow lumen of the shunt. Normotension was soon established by rapid opening up of portosystemic collaterals.