His bundle electrography was performed in three patients with bilateral bundle branch block who manifested similar QRS complexes during atrioventricular (A-V) dissociation and intact A-V conduction. In two such patients ventricular depolarizations (V) were not preceded by His bundle potentials (H) at regular H-V intervals, suggesting a ventricular origin for the subsidiary pacemaker. In the third patient, H preceded V at shortened H-V intervals, indicating a proximal bundle branch rhythm with retrograde His bundle depolarization. It is concluded that the presence of similar QRS complexes during A-V conduction and A-V dissociation does not always indicate an A-V junctional origin of the subsidiary pacemaker. Under these conditions, His bundle electrography is a useful method for determining the site of lower impulse formation.