The effects of a moderate sized arterio-venous shunt on the relationship of oxygen supply to consumption were studied on a regional basis in the dog left ventricle. Experiments were conducted on fourteen hearts, seven in control pentobarbital-anaesthetised, open-chest dogs and seven in dogs with an additional abdominal aorto-caval shunt. Regional oxygen extraction was determined by a microspectrophotometric method and coronary blood flow was measured with radioactive microspheres. The shunt, when opened, allowed cardiac output to increase by 50%. This was accomplished by increasing stroke volume, not heart rate. In control animals the subendocardium exhibited higher oxygen extraction, coronary flow, and oxygen consumption than did the subepicardium. There were no differences between base and apex. In open-shunt animals, subepicardial-subendocardial differences in extraction, flow and consumption were abolished. Subepicardial values for these in shunt animals were not different from the values obtained for the subepicardial values of controls. The subendocardium of shunt animals exhibited significant differences between base and apex for blood flow and oxygen consumption. There was a significant difference in subendocardial oxygen consumption between groups, the effect of the arterio-venous shunt being to reduce subendocardial consumption, especially in the apex, without reducing subepicardial oxygen consumption.