Cardiac catheters were placed in seven healthy conscious patients so that aortic and left ventricular pressures (and the derivative), cardiac output (thermodilution) and myocardial blood flow (argon washin) could be measured. Blood was drawn for measurement of arterial blood-gas and arterial and coronary venous oxygen, glucose, lactate, pyruvate and fatty acid values. After induction of anesthesia by inhalation of halothane, the measurements were made during administration of low (0.70 per cent) and high (1.54 per cent) end-tidal halothane concentrations. Myocardial function decreased in a dose-related fashion without a change in heart rate. Myocardial blood flow and oxygen consumption were depressed in a similar manner. Myocardial oxygen extraction decreased and lactate did not change, suggesting that myocardial oxygenation was adequate. The heart rate-systolic blood pressure product correlated poorly with myocardial oxygen consumption. Systolic blood pressure and the contractile performance index dP/dt/IP were better correlated with myocardial oxygen consumption, but the value of the coefficient was still low. Without significant changes in heart rate, systolic blood pressure is the best correlate of myocardial oxygen consumption in healthy man during the myocardial depression produced by halothane.