27 cardiac patients with different types of arrhythmias were treated in the coronary care unit with tiapamil administered by intravenous infusion. Special attention was given to determining the antiarrhythmic effects in patients with acute coronary insufficiency. In the group of 15 patients with ventricular extrasystoles (VEs; Lown classification III-V), 9 patients had acute myocardial infarction and 1 patient had coronary artery disease. In the group with supraventricular extrasystoles (6 cases), 1 patient had experienced anterior infarction. The third group comprised 6 patients with combined ventricular (Lown V) and supraventricular extrasystoles. In 1 case, the arrhythmia was due to acute anterior infarction, 2 patients had experienced reinfarction and one patient had crescendo angina pectoris. ECG and hemodynamics were monitored continuously before, during and for 20 h following therapy. In patients with VEs alone, the median frequency fell from 612 to 459/h at the third hour of infusion. The median VE/sinus beat quotient decreased from 0.125 to 0.0108 (p < 0.01) in the third hour of treatment, and increased to 0.1029 after the completion of therapy. VE suppression was particularly marked in the 10 patients with coronary artery disease, 7 of these demonstrating a reduction by over 90%. Similar results were obtained in patients with supraventricular arrhythmias and mixed forms.The preliminary results show that tiapamil may be an effective antiarrhythmic agent in supraventricular and particularly in ventricular extrasystoles, and that its spectrum of antiarrhythmic action merits detailed investigation in larger numbers of patients.