A comparison of the effects of several antiarrhythmic agents was made in a study of 70 patients - 15 with manifest Wolff-Parkinson-White (WPW) syndrome, 17 with concealed WPW syndrome, 18 with AV nodal re-entrant tachycardia, 14 with paroxysmal atrial fibrillation and 6 with paroxysmal atrial flutter - employing intracardiac stimulation and esophageal pacing. For the termination of paroxysmal supraventricular tachycardia, intravenous administration of verapamil or aprindine was more effective than that of disopyramide or procainamide. In AV nodal re-entrant tachycardia, verapamil was the most effective for termination. In the manifest WPW syndrome, disopyramide or aprindine was indicated especially for patients with the accessory pathways of the short antegrade refractory period, because these drugs lengthened the refractory period of the accessory pathways. For the purpose of converting atrial fibrillation or flutter to the sinus rhythm, type IA drugs such as disopyramide were indicated. However, verapamil was effective for slowing down the ventricular rate in atrial fibrillation or flutter except in cases of manifest WPW syndrome. A 6-month follow-up study showed that oral administration of verapamil was also useful for putting a stop to the attacks in 24 out of 32 patients with paroxysmal supraventricular tachycardia, while oral disopyramide prevented the recurrence of atrial fibrillation in only 4 of 10 patients.