Myocardial necrosis from repeated direct current defibrillation discharges is less when the same stored energy is delivered by paddle electrodes that are larger than those presently available on the majority of commercial defibrillators. The present study was undertaken to determine if the larger 12.8 cm. diameter paddle electrodes are as effective as the standard 8.0 cm. diameter paddle electrodes in defibrillation. Ventricular fibrillation (VF) was induced in 45 dogs and each was allowed to remain in ventricular fibrillation for progressively longer time intervals before defibrillation was attempted. With the 12.8 cm. diameter paddle electrodes, the longest duration of VF sucessfully terminated was 1.22+/-1.05 minutes when the 8.0 cm. paddle electrodes were used (p less than 0.02). Ventricular fibrillation was terminated during the first minute with the 12.8 cm. diameter electrode in 88 per cent of trials as compared with a 71 per cent success rate with 8.0 cm. diameter paddle electrodes (p less than 0.04). When the success rates during the first minute of VF for both sizes of paddle electrodes were ploted against the measure transthoracic impedance, a high correlation cofficient (r=-0.94) was found. This study suggest that 12.8 cm. diameter paddle electrodes are more effective for defibrillation of subjects in the 13.5 kilogram (29 to 69 pound) weight range than are paddle electrodes that are only 8.0 cm. in diameter.