The efficacy of intravenously administered tiapamil in treating premature ventricular contractions (PVCs) in the acute phase of acute myocardial infarction was evaluated in 20 patients using 24-hour continuous ECG monitoring (Holier). All patients presented frequent and/or complex ventricular arrhythmias (Lown classification II-V). Tiapamil reduced the number of PVCs to a statistically highly significant degree (p < 0.01), i.e. 82-85% in the 4th hour of treatment. The number of PVCs increased again after therapy was discontinued. Where supraventricular ectopic beats were present in addition to PVCs, tiapamil reduced them by 93% in the 3rd and 4th hours of treatment. These results suggest that tiapamil could be an effective antiarrhythmic agent in acute myocardial infarction.