This study was designed to test the hypothesis that the effects of a strong electrical stimulus on reentrant wavefronts in ventricular fibrillation (VF) are dependent on the timing of the stimulus. We studied six open-chest dogs by computerized mapping techniques. A plaque electrode array with up to 509 bipolar electrodes was placed on the right ventricular epicardium. The interelectrode distance was 1.6 mm, and the interpolar distance was 0.5 mm. After eight baseline pacing stimuli (S1) with cycle lengths of 300 ms, a strong premature stimulus (S2) (73 +/- 10 mA) was given to induce VF. In subsequent episodes, a second strong premature stimulus (S3) was given at progressively longer S2-S3 intervals in 20-ms increments. The results showed that, at baseline, the S2 consistently induced figure-eight reentry and VF. The VF cycle length immediately after the S2 averaged 108 +/- 17 ms. The S3 resulted in one of the following responses: (1) termination of reentry and VF; (2) induction of different reentrant wavefronts or a focal pattern of repetitive activation; or (3) persistence of the same figure-eight reentry. The intervals between the S3 and the immediately preceding activation at the site of the S3 (the recovery intervals) were 39 +/- 12 ms (range, 20 to 60 ms) and 61 +/- 20 ms (range, 30 to 90 ms) for response patterns 1 and 2, respectively. The recovery intervals associated with response pattern 3 were either < or = 30 ms (22 +/- 8 ms) or > or = 80 ms (94 +/- 15 ms). The differences among these four intervals were significant (P < .001). We conclude that the effects of strong electrical stimulation on the reentrant wavefronts in VF are dependent on the recovery interval since the previous local activation. A protective zone occurred between 20 and 60 ms, during which time a strong electrical stimulus could terminate reentry and abort VF. This zone was followed by a vulnerable period during which new activation wavefronts could be induced. If a strong electrical stimulus was given shortly after or sufficiently long after the previous local activation, the same figure-eight reentrant pattern continued.