The clinical and electrophysiologic effect of intravenous dofetilide was evaluated in patients with paroxysmal atrial fibrillation (AF) of recent onset (< 7 days) and paroxysmal supraventricular tachycardia (PSVT). From 2.5 to 5.0 micrograms/kg of dofetilide was administered intravenously for the termination of arrhythmias. For the electrophysiologic study (EPS), 3.0 micrograms for loading and subsequently 2 micrograms/kg was injected for 45 min as a maintenance dose. The EPSs were performed before the loading and during the maintenance dose. AF was successfully converted to sinus rhythm in seven (54%) of 13 patients. The duration of AF from its onset was significantly shorter in responders than that of nonresponders (p < 0.05). Dofetilide also terminated PSVT in four of six patients. In the EPS, dofetilide proportionately lengthened the effective refractory period of the atrium, ventricle, and the accessory pathways without slowing of the intracardiac conduction. Dofetilide completely suppressed the induction of PSVT in seven of 13 patients, restricted the induction zone in five, and inhibited perpetuation of the arrhythmia in the remaining one. The cycle length of PSVT remained unchanged after dofetilide. These results imply that the suppression of the development and maintenance of reentrant arrhythmias may result from the lengthening effect of dofetilide on the refractoriness and the consequent elimination of the excitable gap at the critical part of the reentrant loop.