1. Experiments were performed on dogs, anaesthetized with pentobarbitone sodium to measure the oxygen utilization of anterior ventricular wall. The anterior interventricular vein was used for venous blood collection and flow measurement. 2. Oxygen contents of arterial blood and coronary vein were measured. 3. Intravenous (femoral vein) adrenaline infusions raised arterial blood pressure and caused an initial fall in oxygen extraction from control values of 67-45%. Noradrenaline caused an initial fall from 67 to 51%. With both, the oxygen extraction began to rise during the infusion and continued, reaching maximum levels, (mean -82%) 10 min after start of infusion. The range of maximum values for oxygen extraction ratio after noradrenaline was 76-95%. 4. An inverse linear relation was established between blood pressure and oxygen extraction; whereas a direct linear relationship was established between coronary venous blood flow and mean arterial pressure. 5. When a change in blood pressure was prevented, the oxygen extraction ratio rose. 6. In all experiments, whether extraction rose or fell, the oxygen consumption rose. 7. It is suggested that oxygen extraction is a controlled variable and, with coronary blood flow, contributes quantitatively to coronary vascular reserve. 8. The tension-time index was not directly related to oxygen consumption in these experiments. Oxygen consumption rose in response to catecholamines by about the same amount even when mean arterial blood pressure was controlled. It is suggested that cardiac adjustments to altered afterload may also involve alterations in mechanical efficiency.