OBJECTIVE Suddenly occurring ventricular tachyarrhythmias are a complication during off-pump coronary artery bypass (OPCAB) surgery, potentially leading to the need for conversion to on-pump surgery. We examined serial changes in the spatial dispersion of the electrical activity and refractoriness at the myocardial ischemia border zones during and after coronary occlusion. METHODS Unipolar epicardial electrograms were continuously recorded from the anterior left ventricle at the border zones during and after a 10-min occlusion of the left anterior descending (LAD) coronary artery in 22 patients undergoing OPCAB. The local electrogram amplitude and local refractoriness were evaluated by the unipolar peak-to-peak amplitude (UPPA) and activation recovery interval (ARI), respectively. The spatial dispersion of the electrical activity and refractoriness were examined using the coefficient of variation of these parameters. RESULTS No sustained ventricular tachyarrhythmias occurred in any patients. The UPPA dispersion significantly increased up to 5 min after the LAD occlusion and then returned to a nonsignificant level and again increased after reperfusion. The ARI dispersion gradually increased after the LAD occlusion, reached a significantly increased level 3 min after the occlusion, and stayed at a significantly increased level for at least 5 min after the reperfusion. CONCLUSIONS There were unique serial changes in the spatial dispersion of the electrical activity and refractoriness at the myocardial ischemia border zones during and after coronary occlusions. Continuous monitoring of these parameters may be useful for predicting the critical electrophysiological conditions prone to the occurrence of ventricular tachyarrhythmias in patients undergoing OPCAB.