A reduction in the shock strength required for defibrillation would allow use of a smaller automatic implantable cardioverter-defibrillator and would reduce the possibility of myocardial damage by the shock. Most internal defibrillation electrodes require 5 to 25 J for successful defibrillation in human beings and in dogs. In an attempt to lower the shock strength needed for defibrillation, we designed two large titanium defibrillation patch electrodes that were contoured to fit over the right and left ventricles of the dog heart, covering areas of approximately 33 and 39 cm2, respectively. In six anesthetized open-chest dogs, the electrodes were secured directly to the epicardium and ventricular fibrillation was induced by 60 Hz alternating current. Truncated exponential monophasic and biphasic shocks were given 10 sec later and defibrillation thresholds (DFTs) were determined. The DFT was 159 ± 48 V, 3.2 ± 1.9 J (mean ± SD) for 10 msec monophasic shocks and 106 ± 22 V, 1.3 ± 0.4 J, for biphasic shocks with both phase durations equal to 5 msec (5-5 msec). The experiment was repeated in another six dogs in which the electrodes were secured to the pericardium. The mean DFT was not significantly higher than that for the electrodes on the epicardium: 165 ± 27 V, 3.1 ± 1.2 J for 10 msec monophasic shocks and 116 ± 19 V, 1.6 ± 0.5 J for 5-5 msec biphasic shocks. Low DFTs were also obtained with biphasic shocks in which the duration of the first phase was longer than that of the second. In a third group of six dogs, DFTs were determined for the large contoured electrodes as well as for 10 cm2 flat patch electrodes on the right and left ventricular epicardium. The mean DFT was significantly lower for the 5-5 msec biphasic waveform than for the 10 msec monophasic waveform for both types of electrodes, and the mean DFT for the large contoured electrodes was significantly lower than that for the flat patch electrodes for both types of waveforms. We conclude that the shock strength required for defibrillation can be markedly lowered by means of biphasic shocks and large contoured patch electrodes.