While it has been shown that pacing during ventricular fibrillation (VF) can capture a portion of the epicardium, little is known about the characteristics of the area captured or about whether adaptively changing the pacing rate during VF will increase the area captured. In six open-chested pigs, pacing during VF was performed from the center of a plaque containing 504 electrodes 2 mm apart in a21 x 24 array on the anterior right ventricle. Simultaneous recordings from the 504 electrodes were used to construct activation maps from which the area of epicardium captured by pacing was determined. Four pacing algorithms were examined: (1) fixed rate pacing at 95% of the median VF activation rate, (2 and 3) adaptive pacing in which the pacing timing and/or rate is reset in real time if capture is not obtained, and (4) pacing at a slowly increasing rate after initial capture. Regional capture, defined as control of the myocardium under at least 10 plaque electrodes, was achieved in 71% (92/129) of pacing episodes. The incidence of capture was not significantly different for pacing algorithms 1-3. The maximum area captured for each pacing episode with algorithms 1-3 was 3.8 +/- 2.0 cm2(mean +/- SD). Within each animal, the pattern of capture was similar among all pacing episodes, no matter which algorithm was use dr = 0.85 +/- 0.25). The region of greatest capture extended away from the pacing site along the long axis of the myocardial fibers. However, the area of captured epicardium toward the right ventricular side of the pacing electrode was 9.7 times greater than toward the left ventricular side. This principal direction toward the right ventricular side of the pacing electrode was the same direction traveled by the majority of VF activation fronts before capture occurred. The absence of recorded activations at the pacing site for 20 consecutive stimuli predicted 83% of the time that regional capture was present. With algorithm 4, the pacing rate could be increased 7.1%+/- 4.3%while maintaining capture; however, the area of capture progressively decreased as the pacing rate increased. While pacing from the anterior right ventricular epicardium during VF, the area of capture is repeatable and is markedly asymmetrical with almost 10 times as much epicardium captured on the side of the pacing electrode closest to the acute margin of the right ventricle as on the opposite side. This marked asymmetry is associated both with myofiber orientation and with the direction of spread of activation and hence the direction of dispersion of refractoriness during VF just before pacing is initiated. It is possible to perform adaptive pacing algorithms in real time during VF; however, the two adaptive algorithms tested did not capture significantly more epicardium than a simple fixed-rate pacing algorithm. Although it is possible to maintain capture while increasing the pacing rate during VF, the area of capture decreases.