The components of the reentrant circuit were evaluated in 26 patients in whom sustained ventricular tachycardia could be reproducibly initiated or terminated, or both. Observations suggesting that the proximal His-Purkinje system was not a requisite component included (1) lack of requirement for retrograde His-Purkinje delay or bundle branck reentry, or both, for initiation of the tachycardia: (2) anterograde depolarization of the His bundle during ventricular tachycardial without alteration of the QRS configuration or cycle length; and (3) the presence of random retrograde His potentials during the tachycardia. Evidence that the reentrant circuit was localized to a small area of the ventricles included (1) the ability to capture large segments of the ventricles transiently or continuously with occurrence of intermittent or continuous supraventricular capture either spontaneously or with atrial pacing without effect on the tachycardia. These findings suggest that the reentrant circuit must be small, electrocardiographically silent and relatively protected.