We compared single to sequential pulse shocks with different pulse separations on internal cardiac defibrillation by using a catheter and plaque electrodes in open-chest halothane-anesthetized pigs. Ten seconds after fibrillation onset, defibrillation was attempted using trapezoidal pulses of 65% tilt, approximately 5 ms duration and fixed outputs from 1.0 to 50 joules (J). With single pulses, minimum defibrillation energy for the catheter alone was 2.4 +/- 0.3 J/kg (mean +/- standard error) and 2.1 +/- 0.2 J/kg for the catheter tip to plaque configuration. With sequential pulse shocks, the first pulse delivered via the catheter and the second pulse from the catheter tip to the plaque electrode, the energy necessary for defibrillation was dependent on the separation time between the two pulses (2.0 +/- 0.2, 1.5 +/- 0.2, 0.9 +/- 0.1, 1.3 +/- 0.3, 0.6 +/- 0.2, and 1.2 +/- 0.2 J/kg at 100, 10, 1, 0.5, 0.2, and 0.1 ms, respectively). Further, at the 0.2 ms separation, 100% of the animals could be defibrillated with less than 2.0 J/kg (35 J total). We conclude that sequential pulse defibrillation provides a significant improvement over single pulse defibrillation. The optimum separation between the sequential pulses in this study was 0.2 ms.