3 patients with paroxysmal palpitations had surface electrocardiographic evidence of type B preexcitation with a small positive delta wave in lead V1. At study all patients had an abnormally short H-V interval. Programmed atrial premature stimuli at progressively shorter coupling intervals resulted in prolongation of AV conduction time and increasing preexcitation. In 2 patients early atrial premature stimuli elicited atrial echo beats or sustained tachycardia with preexcited QRS complexes. In these patients the accessory pathway did not appear to be involved in the tachycardia circuit. It is postulated that the site of reentry is within the AV node with preexcitation occurring as the result of conduction in an anomalous node-ventricular pathway. In the 3rd patient, preexcitation occurred within a restricted range of coupling intervals of an atrial premature stimulus. The electrophysiological responses of nodo-ventricular and His-ventricular pathways to atrial stimulation are compared and contrasted.