Currently available internal cardiac defibrillators use a uniphasic, truncated exponential waveform morphology of about 6 msec in duration at an energy level of 23 to 33 joules. To determine if improved defibrillation could be achieved with a different waveform morphology, we implanted 4.5 cm2 titanium patches to the left and right ventricle of 28 dogs. After ventricular fibrillation was induced, defibrillation was attempted using 7, 12, 13, or 17 joules. A 5 msec rectangular uniphasic waveform morphology was compared with a 10 msec rectangular biphasic waveform with the lagging 5 msec pulse of half the amplitude of the leading 5 msec. In an additional seven dogs, a transvenous bipolar catheter was placed with the distal electrode in the right ventricular apex and the proximal electrode in the superior vena cava. Biphasic and uniphasic shocks were compared at 14 joules. In the patch-patch system, the biphasic waveform was superior to the uniphasic waveform at 7 joules (67% versus 35%, p less than 0.001) and at 12 joules (93% versus 78%, p less than 0.001). No statistically significant differences were achieved at 13 joules or 17 joules. In the catheter electrode system with a delivered energy of 14 joules, the biphasic waveform was more effective than the uniphasic waveform (87% versus 27%, p less than 0.001). Manufacturers of automatic implantable defibrillators should consider this information in the design of future automatic implantable defibrillators.