The extension of refractoriness hypothesis predicts that defibrillation success can be improved by synchronizing shocks to action potentials from cells in low intensity regions. Threshold should be lower for shocks delivered "early" in these action potentials than for shocks delivered "late". To test this hypothesis, defibrillating shocks were synchronized to monophasic action potentials recorded from a low intensity region in isolated rabbit hearts (N=6). Probability-of-success curves were generated for "early" and "late" shocks. The authors found that I/sub 50/ for "early" shocks was 17% lower than I/sub 50/ for "late" shocks (p<0.05). This corresponds to a 31% decrease in E/sub 50/ The relative standard deviation of I/sub 50/ decreased from 32% for "late" shocks to 18% for "early" shocks. Standard deviations at all success percentiles were larger for "late" shocks. These results suggest that synchronizing shocks to action potentials from a low intensity region may reduce defibrillation threshold and inter-subject variability.