The threat of reperfusion fibrillation could potentially deter attempts of reperfusion in patients with acute myocardial infarction. Delineation of the mechanisms of this arrhythmia could pave the way to newer interventions designed as prevention or definitive treatment. Therefore the purpose of the present study was to investigate the features of initiation of reperfusion-induced ventricular fibrillation, and further to distinguish between episodic and sustained ventricular arrhythmias following reperfusion. Nine instances of reperfusion ventricular fibrillation and five instances of episodic tachyarrhythmia were analyzed in the study utilizing endocardial bipolar electrograms from normal, ischemic, reperfused, and border of these myocardial segments. In 11 of 14 instances, the site of initiation of the tachyarrhythmias was in the reperfused myocardium; however, maintenance of the arrhythmia defined as diastolic and or continuous electrical activity suggestive of reentry was not seen in the reperfused myocardium in any of these instances. Diastolic electrical activity was observed in 8 of 14 instances, and was seen either in the center or border of ischemic myocardium. Neither heart rate or mean aortic pressure was different between episodic and sustained arrhythmia groups; however, the initial beat of the fatal arrhythmia was significantly more premature than that of episodic arrhythmia. In addition, shorter cycle length, greater variations in the cycle length, and greater disparity in local activation during the tachyarrhythmia were seen in the sustained arrhythmia group compared to the episodic group; cycle length increased and the disparity in local activation improved gradually prior to the termination of the arrhythmia. There was no particular difference in conduction delay immediately prior to reperfusion between control and reperfusion ventricular fibrillation groups. We conclude that different mechanisms exist for the initiation and maintenance of reperfusion-induced arrhythmias. Further, several features seem to distinguish episodic from fatal arrhythmias following reperfusion.