Automatic defibrillators have been successfully tested in normal animals. However, human candidates for implantation of such devices are likely to have ischemic heart disease. This study examined the optimal site of defibrillation and the influence of acute myocardial ischemia upon the defibrillation threshold in anesthetized dogs. The defibrillation threshold was determined from a transvenous right ventricular intracavitary electrode and from right and left ventricular epicardial electrodes. Shocks were delivered before and after occlusion of the left anterior descending coronary artery. Before occlusion, the rate of successful shocks was low from the right ventricular epicardium, moderate from the right ventricular cavity, and high from the left ventricular epicardium. Furthermore, the defibrillation threshold was significantly lower at the left ventricular epicardium than at the right ventricular sites. During coronary artery occlusion, the rate of successful defibrillation remained high from the left ventricular epicardium, and there was no significant change in the defibrillation threshold. It was concluded that the left ventricular epicardium is the optimal site for defibrillation in the anesthetized dog. Acute coronary artery occlusion did not modify the success rate of defibrillation or the energy required for defibrillation.