In the differential diagnosis of tachycardias showing a wide QRS complex and having a 1 to 1 relation between ventricular and atrial events, a supraventricular tachycardia with anterograde conduction over an accessory pathway and retrograde conduction by way of the specific conduction system must be considered. Five patients showing this type of circus movement tachycardia were studied by programmed electrical stimulation of the heart. Sudden changes in the tachycardia cycle length were observed in these patients that were based on changes in the VH interval. This finding suggested a change in the reentrant circuit with anterograde conduction over the accessory pathway but retrograde conduction sometimes occurring over the right bundle branch and at other times over one of the two divisions of the left bundle branch system. Characteristically, the tachycardia cycle length changed suddenly depending on the bundle branch used in retrograde direction. In one patient, an important difference was also observed between the anterograde effective refractory period of the accessory bypass (280 ms) and the shortest RR interval between preexcited QRS complexes during atrial fibrillation (measuring 190 ms). It is postulated that the short RR intervals during atrial fibrillation in the Wolff-Parkinson-White syndrome could result from bundle branch reentry after activation of the ventricles over the accessory pathway.