Circulatory and respiratory effects of change of posture from sitting In supine were studied in 11 obese patients scheduled for gastric bypass operations. Patients were studied on the morning of operation before any medication was given. Average weight was 138.8 kg, 108.6 per cent above ideal weight. Change of posture from sitting to supine was associated with II per cent increase in oxygen consumption (Vo2, 35.5 per cent increase in cardiac output (CO)2, 35.8 per cent increase in cardiac index, 17.85 per cent decrease in arteriovenous oxygen difference, 31 per cent increase in mean pulmonary arterial pressure. 44 per cent increase in pulmonaryartery wedge pressure. 21.5 per cent decrease in peripheral resistance (PR), 6 per cent decrease in heart rate, and 17.7 per cent increase in venous admixture (Qs/QtAll these changes were significant. There was no change in mean arterial pressure, alveolo-arterial O2difference and respiratory rate. The increase in CO was attributed to reduction in PR and to increased V02probably due to increased work of breathing.