Eight of 120 consecutive patients with inducible sustained ventricular tachycardia who were studied at our institution from September 1, 1988 to January 1, 1991, were found to have reentry within the His-Purkinje System as the mechanism of their tachycardias. Two of the eight patients (25%) required the recording of the right bundle branch potential to elucidate the tachycardia circuits. The electrophysiological findings of these two patients are described. In both instances, the diagnosis of supraventricular tachycardia with aberrancy was excluded. In patient 1, a His-bundle electrogram preceded each QRS complex during tachycardia and the His-to-His interval variation preceded changes in QRS intervals. However, recordings from the right bundle branch allowed for exclusion of bundle branch reentry and evidence was found for reentry restricted to the left fascicles. In patient 2, despite instances of dissociation of the His-bundle deflection from the tachycardia, a right bundle branch potential preceded each QRS and spontaneous changes in the interval between successive activation of the right bundle branch preceded changes in ventricular activation. Catheter ablation of the right bundle branch eliminated the tachycardia. It is concluded that the recording of a right bundle branch potential should be included in electrophysiology study of patients in whom there is suspicion of reentry within the His-Purkinje System. Clinically, recognizing these forms of tachycardias can be important because they can be effectively treated with catheter ablation.