ABSTRACTTo examine the relationship between the defibrillation threshold and the strength of shocks that induce ventricular fibrillation during the vulnerable period, we determined the defibrillation threshold in 22 open-chest dogs using epicardial defibrillation electrodes with the cathode at the ventricular apex and the anode at the right atrium. We also determined whether there was an upper limit of shock strength that induces fibrillation in the vulnerable period by giving shocks of various energy through these same electrodes during the repolarization phase of paced rhythm. The above determinations were also made with the anode at the ventricular apex and the cathode at the right atrium in eight of the dogs and with the cathode at the ventricular apex and the anode at the left atrium in another eight of the dogs. In all dogs for all electrode configurations, there was an upper limit to the shock strength that induced ventricular fibrillation during the vulnerable period. Depending on the electrode combination, this upper limit of ventricular vulnerability either was not significantly different from or was slightly lower than the defibrillation threshold. The correlation coefficient between the two was highly significant for all three electrode configurations. These results support the hypothesis that successful defibrillation with epicardial electrodes requires a shock strength that reaches or exceeds the upper limit of ventricular vulnerability and that shocks slightly lower than the defibrillation threshold fail because they reinitiate ventricular fibrillation by stimulating portions of the myocardium during their vulnerable period.