The reentrant wave fronts in ventricular fibrillation (VF) have only a limited life span. The mechanisms by which these reentrant wave fronts terminate are unknown. We performed computerized mapping studies in six open-chest dogs before and after right ventricular subendocardial ablation with Lugol's solution. Recordings were made with 56 bipolar electrodes separated by 3 mm. Baseline pacing was performed on the right side of the tissue to create parallel activation wave fronts. A premature 50-V shock of either anodal or cathodal polarity was given to a bar electrode on the upper edge of the tissue. Counterclockwise reentrant wave fronts and VF were induced both before (60 episodes) and after (57 episodes) subendocardial ablation with either anodal or cathodal shocks. Among these reentrant wave fronts, 8 episodes before and 10 episodes after ablation had over 10 rotations (P = NS). The reentrant wave fronts in other episodes terminated with an average of 3.2 +/- 1.9 rotations before and 3.1 +/- 1.8 rotations after the ablation (P = NS). The reentrant wave-front cycle length was 118 +/- 19 milliseconds before and 124 +/- 20 milliseconds after ablation (P = .001). Conduction block occurred when the wave front was traveling across the myocardial fibers. When conduction was blocked in these episodes, the leading edge of the reentrant wave front encountered tissue that had been excited within the past 58 +/- 12 milliseconds (range, 28 to 77 milliseconds), which corresponded to 47 +/- 12% of the preceding VF cycle length. This period was significantly shorter than the recovery period in the same region that had allowed conduction (91 +/- 19 milliseconds; range, 48 to 137 milliseconds), which corresponded to 72 +/- 18% of the preceding VF cycle length (P < .001). In nine episodes, reentrant wave-front activity terminated when wave fronts that had originated from outside the mapped tissue interfered with the reentrant pathways. Conclusions are as follows: (1) The refractory period of fibrillating ventricular muscle ranges from 48 to 77 milliseconds. Because the refractory period is much shorter than the VF cycle length, a large excitable gap is present in the reentrant circuit. The presence of a large excitable gap contributes to reentrant wave-front termination. (2) Myocardial fiber orientation is an important determinant of the site of conduction block. (3) Although subendocardial ablation slowed the wave-front propagation, it did not prevent the generation and the maintenance of reentry and VF.