Introduction: This study tested the hypothesis that the defibrillation threshold (DFT) can be lowered by delivering a weak auxiliary shock in conjunction with a stronger primary shock to the cardiac region where the primary shock electric field is weakest. Methods and Results: Eight swine were studied in each of two study parts. In both parts, DFTs were determined for dual shocks delivered through two electrode pairs. The biphasic primary shock was delivered through electrodes in the right ventricle and superior vena cava. The auxiliary shock was delivered through a separate electrode in the superior vena cava and a left ventricular electrode placed where the primary shock field was presumed to be weakest. In part I, a monophasic auxiliary shock of 50, 100, or 150 V was delivered either simultaneously with or 1, 20, or 40 msec before primary shock. When auxiliary shock was delivered simultaneously with or 1 msec before primary shock, DFT energy was reduced by approximately 50% compared with primary shock alone. In part II, a 150-V monophasic or biphasic auxiliary shock of either polarity was delivered 1 msec before or after primary shock. Regardless of waveform or polarity, all auxiliary shock delivered before primary shock lowered DFT energy by approximately 30% compared with primary shock alone. Depending on waveform and polarity, auxiliary shock delivered after primary shock either did not significantly change the DFT or elevated the DFT compared with primary shock alone. Conclusion: Application of a small auxiliary shock, just before or simultaneously with a primary shock, to the cardiac region where the primary shock field is weakest significantly lowers DFT.