The effects of atrial pacing on the relationship of O2 supply to demand were studied on a regional basis in the dog heart. Experiments were conducted on fourteen hearts, seven in control pentobarbital-anesthetized, open-chest dogs and seven in dogs paced at 225 beats/min. Regional O2 extraction was determined by a microspectrophotometric method, and coronary blood flow was measured with radioactive microspheres. There was an 89% increase in coronary blood flow, a small but significant decrease in O2 extraction, and an increase in O2 consumption with atrial pacing. With pacing the increase in blood flow and the decrease in O2 extraction were uniform throughout the heart. In control hearts, right ventricular blood flow and O2 consumption were lower than the mean in the septal or left ventricular free wall. In the left ventricular free wall in control, the subepicardial region had a lower O2 consumption and O2 extraction than the subendocardial region. These differences were also preserved during pacing. Due to the large increase in coronary blood flow, the relationship of oxygen supply to demand was improved by atrial pacing.