This study evaluated the energy requirements for porous electrodes implanted on the parietal pericardium versus those for porous electrodes implanted on the epicardial surface of the heart. Defibrillation with a 2.5-cm porous electrode implanted on the parietal pericardium was successful in 42% of all episodes of ventricular fibrillation. The minimal energy requirement ranged from 30-68 J, with an average of 46 J. Epicardial defibrillation was successful in approximately 80% of all episodes. The average maximal energy was 21.3 J. There was histological evidence of subepicardial damage in the parietal defibrillation group. This may be related to the higher energy required to defibrillate in this group. However, in this group frequent, large, external shocks were required to defibrillate the dogs' hearts. Theoretically, there may be some physiological and surgical advantages to an intact pericardium. However, the high energy requirement and the low success rate for defibrillation with a porous electrode on the parietal pericardium negate the feasibility of this route of defibrillation with a permanent implantable system.