Although the mechanism of most episodes of recurrent ventricular tachycardia (VT) is believed to be re-entry, definitive proof of this hypothesis has heretofore been unavailable in man. Using programmed stimulation and ventricular endocardial mapping we studied three patients in whom the initiation of VT was dependent upon developing a critical degree of fractionation and delay in local left ventricular electrograms. When electrical activity spanned diastole, VT ensued. Maintenance of VT was associated with continuous electrical activity resembling "local fibrillation" while termination of VT required cessation of this continuous activity. During sinus rhythm the electrogram recorded from the areas which subsequently developed continuous activity showed markedly fractionated and prolonged electrical activity exceeding 100 msec in duration. We feel these observations of the temporal relationship of continuous activity with the development of VT represent the first documentation of the re-entrant nature of this arrhythmia in man.