Three patients with recurrent supraventricular tachycardia were presented. Atrial cycle length unchanged during the tachycardia with antegrade Wenckebach AH block was observed. When AH block occurred during tachycardia, the first AH interval was shorter than the subsequent one. The tachycardia was initiated and terminated by atrial extrastimulation beyond the atrial relative refractory period and the atrial activation sequence during the tachycardia was low to high. The induction of tachycardia was dependent on a critical AH interval. Ventriculoatrial conduction was not observed in patient 1 and 2. In patient 3 who had ventriculoatrial conduction, the tachycardia was initiated by the premature ventricular stimulation followed by double atrial response, and the tachycardia was terminated by the ventricular stimulation without atrial capture. In patient 1, verapamil (5 mg) prolonged the atrial cycle length during tachycardia and rapid intravenous injection of adenosine triphosphate (10 mg) terminated the tachycardia. Oral diltiazem (180 mg/day) suppressed the tachycardia in patients 2 and 3. These findings suggest that the mechanism of the tachycardia may be fast-slow type of AV nodal reentry in the upper portion of the AV node and this type of arrhythmia has a tendency to be incessant.