The antiarrhythmic effect of oral Mexiletine was evaluated by Holter monitoring on 10 subjects with chronic high-frequency ventricular premature complexes and ventricular tachycardia referred to our cardiology clinic of Severance Hospital from June, 1982 through September, 1983. The frequency of the ventricular premature complexes and the ventricular tachycardia was measured by 24-hour ambulatory electrocardiography on each patient before and during the administration of Mexiletine(450-600 mg/day). The results were as follows : 1) In 10 patients with ventricular tachycardia total suppression of ventricular tachycardial was demonstrated in 8 patients during the administration of oral Mexilletine. 2) The number of the ventricular premature complexes was reduced markedly in 6 out of the 10 patients. 3) Side effects occurred in 5 out of 10 patients. These include tremor, weakness, dry mouth, indigestion, anorexia, chest discomfort, and dizziness but were tolerated except in one.