The antiarrhythmic effects of tiapamil were investigated in 48 patients, mean age 54, with acute myocardial infarction and ventricular arrhythmias. 9 had ventricular tachycardia, 6 ventricular fibrillation, and 33 multifocal premature ventricular contractions. Before and after therapy, ECG parameters, left ventricular ejection fraction (measured using <sup>99m</sup>Tc-labelled blood cells), and serum enzymes were recorded. Tiapamil was administered in a dose of 1 mg/kg i.v. (injection) followed by 25 µg/kg/min i.v. (infusion) until a 70% reduction in premature ventricular contractions was achieved. Thereafter, tiapamil was given in oral administration of 200 mg three times daily for 3 months.13 patients did not respond to treatment. Of these, 5 had ventricular tachycardia and 4 ventricular fibrillation. 6 patients died and showed massive necrosis at autopsy. Premature ventricular contractions decreased by 70-90% in the remaining 35 patients (73%). Mean arterial pressure and heart rate decreased significantly, the left ventricular ejection fraction improved, and the P-R interval increased significantly. No side effects were observed. It is concluded that tiapamil has the propensity to control premature ventricular contractions in patients with acute myocardial infarction.