In a patient with atrioventricular nodal reentrant tachycardia, during programmed stimulation, an atrial extrastimulus induced a double ventricular response due to a single atrial depolarization, with simultaneous and delayed anterograde conduction through fast and slow pathways, and induced the tachycardia. Pacing-induced type I block involving both pathways put these pathways out of phase, so that the distal conduction system and the ventricle responded to both the fast and slow pathways anterograde impulses.