Cardiac output, oxygen uptake and plasma catecholamines were studied in patients both awake and during anaesthesia prior to and during upper abdominal surgery. Two different forms of anaesthesia were used: neurolept-nitrous oxide (NLA) and halothane (HALO) anaesthesia. Oxygen uptake was determined by using a masspectrometer, and cardiac output was measured according to the Fick principle. Plasma catecholamines were analysed by high performance liquid chromatography. Cardiac output fell by 40% during NLA and by 30% during HALO. Concomitantly, the oxygen uptake fell by 40% and 35%, respectively. A linear relationship between cardiac output and oxygen uptake could be established both in the awake state and during anaesthesia, with no significant change in the slope or position of the regression line when anaesthesia was commenced. Ventricular filling pressures fell during both anaesthetic procedures. Adrenaline fell to half the plasma concentrations seen in normal subjects under resting conditions, while noradrenaline returned to normal from an initially 30-40% increased value. Surgery caused no significant changes in either cardiac output or oxygen uptake, whereas plasma adrenaline increased by 20 times and noradrenaline by 60-90%. The findings suggest that the reduced oxygen uptake during anaesthesia causes the fall in cardiac output rather than any cardiodepressant action of the anaesthetic. It is possible that the anaesthetic depresses whole-body metabolism by either blocking the effects of catecholamines or interfering with cellular metabolism.