The antiarrhythmic effect of Mexiletine was evaluated in five patients with recurrent ventricular tachycardia and in twelve patients with frequent premature ventricular contractions. Electrophysiologic study was performed on the the patients with recurrent ventricular tachycardia before and after the administration of Mexiletin(600-800mg/day). The antiarrhythmic effects of Mexiletine in the patients with frequent premature ventricular contraction was assessed by ambulatory electrocardiography. The results were as follows: 1) In all of the five patients with recurrent ventricular tachycardia, sustained ventricular tachycardia was induced by programmed ventricular stimulation. However, on repeated electrophysiologic study performed while receiving Mexiletine, ventricular tachycardia was not induced in four patients and non-sustained ventricular tachycardia was induced in one patients. 2) The number of premature ventricular contraction was decreased markedly in seven patients(58.3%) with frequent premature ventricular contractions. 3) Minimal side effects, mild indigestion and tremor, were recorded in 3 cases(23%) while receiving Mexiletine 600 mg/day. The higher dose was associated with more frequent and severe side effects.