Previous studies have demonstrated the efficacy of bepridil, a new calcium antagonist, in the treatment of ventricular extrasystoles and tachycardia. The electrophysiological properties of bepridil especially the lengthening of the atrial effective refractory period, would also suggest an antiarrhythmic effect at the supraventricular level. This effect was studied on 33 episodes of paroxysmal supraventricular tachycardia (SVT) occurring in 23 patients (6 men and 17 women, mean age 58.1 years; range 18 to 88 years). Bepridil was given intravenously over 5 minutes at a dose of 3 mg/kg. The duration of SVT before administration was less than 1 hour in 10 cases, between 1 and 2 hours in 8 cases and over 2 hours in 15 cases. Sinus rhythm was successfully restored in 25 cases: within 1 to 5 minutes in 19 cases, 6 to 10 minutes in 3 cases and 11 to 30 minutes in 3 cases. In 24 of the 25 cases sinus rhythm was restored without a prolonged pause (over 2 sec) after the termination of SVT; in 3 cases intermediary atrial fibrillation lasting 1, 3 and 9 minutes was observed. There were no side-effects in 26 cases; transient flushing was noted in 5 cases and vagal symptoms in 2 cases. Haemodynamic tolerance judged by blood pressure measurements excellent in all cases. The correlations between plasma concentrations of bepridil and success or failure were poor. In conclusion, bepridil is a valuable alternative to adenosine triphosphate which may induce an exaggerated vagal response and to verapamil whose negative inotropic effects may sometimes be a serious disadvantage in the reduction of paroxysmal SVT.