Electrophysiologic study, isoproterenol infusion, and serial treadmill exercise tests before and after administration of propranolol, verapamil, lidocaine, and procainamide were done in three patients with exercise-triggered ventricular tachycardia. In all three patients, organic heart diseases were absent. Ventricular tachycardia was reproducibly provoked with exercise and with isoproterenol infusion. Propranolol (tested in three patients) and lidocaine (tested in two patients) effectively prevented exercise provocation of tachycardia. Verapamil terminated tachycardia in all three patient and successfully prevented exercise provocation of tachycardia in only two patients. Procainamide was ineffective in one patient and was partially effective in two patients. In the latter two patients, ventricular ectopies, couplets, and short salvos remained provocable with exercise. Electrical stimulations with incremental ventricular pacing and ventricular extrastimulus testing failed to induce tachycardia in all three patients. These findings strongly suggest that repetitive rhythmic activities related to the catecholamine-sensitive afterdepolarizations are probably responsible for the exercise-triggered ventricular tachycardia.