15 patients (10 M, 5 F; age 24-55) with paroxysmal supraventricular tachyarrhythmias (onset: 0.5-4 hours; 8 atrial tachycardias, 5 atrial fibrillation, 2 atrial flutter) were treated with 1 mg/kg i.v. propafenone. Propafenone terminated tachyarrhythmias in all patients during or within a few minutes after stopping infusion (mean conversion time: 6.4 min.). No significant changes in the main electrocardiographic parameters (QRS from 67 +/- to 80 +/- 21 msec., n.s.; QTc from 330 +/- 60 to 320 +/- 40 msec, n.s.), nor in blood pressure (from 128/82 +/- 21/10 to 135/83 +/- 11/8 mmHg, n.s.) were observed. No side effects appeared. In conclusion, propafenone proved to be effective and well tolerated in the acute treatment of supraventricular tachyarrhythmias.