Recent technological advances have resulted in high success rates for implantation of nonthoracotomy defibrillation lead systems. Further decreases in defibrillator size, facilitating pectoral placement, will depend in part on lowering defibrillation energy requirements. The purpose of this study was to determine if endocardial defibrillation energy requirements are influenced by electrode size. Thirteen adult mongrel dogs were studied under general anesthesia. A 9 Fr integrated bipolar pace/sense/defibrillation electrode (cathode) was positioned transvenously at the RV apex. The second defibrillation electrode (anode) was positioned at the junction of the RA and SVC. Two diameters of the proximal electrode, 7 Fr and 11 Fr, were sequentially tested in random order in each animal. The DFT for each electrode was determined using a 50-V up-down method. Energy, leading edge voltage, and current, current distribution, and total resistance were measured. The mean defibrillation voltage threshold with the 11 Fr proximal electrode was significantly less than with the 7 Fr proximal electrode (551.1 +/- 76.5 V vs 588.5 +/- 54.6 V, P < 0.01). Similarly, the mean DFT with the 11 Fr electrode was less than with the 7 Fr electrode (20.7 +/- 5.7 J vs 23.3 +/- 4.4 J, P < 0.01). Lower DFTs were found using the larger electrode in 11 of the 13 animals studied. However, there was no difference in defibrillation lead impedance between the two electrode systems. Endocardial defibrillation energy requirements may be lowered with a larger diameter proximal electrode. The mechanism by which this occurs may be due to a more even distribution of current gradients with the larger electrode. Determination of the optimal electrode size requires evaluation in humans, as this may allow further reduction in defibrillation energy requirements and defibrillator size.