18 patients with acute myocardial infarction and sustained supra ventricular arrhythmias were treated with tiapamil, in a dose of 1 mg/kg i.v. The drug was effective, i.e., the heart rate was reduced to less than 90 beats/min in 9 of 10 patients with atrial fibrillation, in 3 of 4 patients with atrial flutter, and in 3 of 4 patients with atrial tachycardia. The peak effect was observed within 2-5 min. In cases with recurring tachyarrhythmias, tiapamil was also effective during successive administrations, the systolic blood pressure was reduced by 10-15%, but severe hypotension was not observed. In these patients, no major changes in atrioventricular or intraventricular conduction were observed. In the dose used, tiapamil was effective and well tolerated.