Reentrant tachycardia occurred after implantation of an atrial synchronous ventricular-inhibited pacemaker in a patient with bradycardia caused by second-degree (Mobitz II) atrioventricular block. Despite the presence of antegrade atrioventricular block, intact ventriculoatrial conduction was present at a cycle length that exceeded the atrial refractory period of the pacemaker. Consequently, a reentrant or "endless-loop" tachycardia occurred. Application of a magnet terminated the tachycardia. Because the episodes were frequent and could not be prevented by medication, the pacemaker was reprogrammed to a ventricular-inhibited mode. All candidates for atrial synchronous pacing should undergo an appropriate electrophysiologic study preoperatively.