In a double-blind placebo controlled study, tocainide, 750 mg i.v. during a 15-minute period directly followed by 800 mg orally and later 400 mg t.i.d., was administered to patients wiht acute myocardial infarction (AMI). Treatment was started as soon as possible following onset of symptoms; the follow-up period was 6 months. The patient groups consisted of 56 tocainide and 56 placebo patients. There was no significant effect on the incidence of ventricular fibrillation or symptomatic ventricular tachycardia. The mortality rates were similar and low in both groups. Tocainide suppressed ventricular arrhythmias, including ventricular tachycardia, both in the acute stage of AMI and during convalescence. Tocainide also suppressed exercies-induced ventricular arrhythmias. Side effects were in general mild or moderate.