Ventricular tachycardia may result in retrograde atrial depolarization. In all previous examples studied by intracardiac recording techniques retrograde conduction to the atria has been thought to occur via the normal conduction system. In the patient reported here, 1 : 1 retrograde conduction with atrial preexcitation during ventricular tachycardia indicated the presence of a concealed accessory pathway. This was confirmed by the phenomenon of exact atrial capture. The achievement of atrioventricular dissociation during tachycardia by atrial overdrive pacing excluded a reentrant tachycardia involving such a pathway. The widely variable H--V interval in the presence of a constant V--A interval during tachycardia suggested anterograde His bundle depolarization following atrial activation over the accessory pathway. This hypothesis was supported by the long V--H interval during tachycardia and the relationship of the ventricular, atrial and His bundle electrograms of the first and least beat of the tachycardia. This report illustrates the value of careful electrophysiological investigation in evaluating the relationship of an accessory pathway to the tachycardia mechanism.