Twelve patients with aortic valve lesions were studied haemodynamically before operation and on the three consecutive days after uncomplicated aortic valve replacement with a Björk-Shiley tilting disc valve prosthesis. Five patients had pure aortic stenosis, 4 aortic insufficiency and 3 had combined lesions. Cardiac output at rest, which was within normal limits before operation, was unchanged on the first postoperative day, but showed a tendency to increase on the two following days. Heart rate was markedly increased postoperatively with a corresponding decrease in stroke volume. Arteriovenous oxygen difference was slightly higher postoperatively, indicating a more hypokinetic circulation after surgery. Right atrial mean pressure, which was normal preoperatively, increased in average 3 mmHg postoperatively, whereas left atrial mean pressure, which was pathologically elevated in most patients before operation, showed a marked decrease after operation with a corresponding decrease in pulmonary artery pressure. No significant change in radial artery pressure was observed. Pulmonary and systemic vascular resistance did not change significantly after surgery. The causes of tachycardia, decreased left atrial pressure and hypokinetic circulation after operation are discussed. It is suggested that, in spite of a positive fluid and blood balance, a relative hypovolaemia exists with insufficient blood volume in the left atrium, leading to diminished filling of the low-compliant left ventricle.