Background - Shocks of identical strength and timing sometimes induce ventricular fibrillation (VFI) and other times do not (NoVFI). To investigate this probabilistic behavior, a shock strength near the upper limit of vulnerability, ULV50, was delivered to yield equal numbers of VFI and NoVFI episodes. Methods and Results - In 6 pigs, a 504-electrode sock was pulled over the ventricles. ULV50 was determined by scanning the T wave. S1 pacing was from the right ventricular apex. Ten S2 shocks of approximate ULV50 strength were delivered at the same S1-S2 coupling interval. Intercycle interval (ICI) and wave front conduction time (WCT) were determined for the first 5 postshock cycles. ICI and the WCT of cycle 1 were not different for VFI versus NoVFI episodes (P = 0.3). Beginning at cycle 2, ICI was shorter and WCT was longer for VFI than NoVFI episodes (P < 0.05). Conclusions - The first cycle after shocks of the same strength (ULV50) delivered at the same time has the same activation pattern regardless of shock outcome. During successive cycles, however, a progressive decrease in ICI and increase in WCT occur during VFI but not NoVFI episodes. These findings suggest shock outcome is (1) deterministic but exquisitely sensitive to differences in electrophysiological state at the time of the shock that are too small to detect or (2) probabilistic and not determined until after the first postshock cycle.