Electrophysiologic and antiarrhythmic effects of the new benzofurane derivative carocainide given intravenously have been studied in normal patients (Group I) and in patients with paroxysmal supraventricular tachycardias (Group II). In normal patients (n = 10), carocainide increased sinoatrial conduction time, PR, PA, AH, and HV interval, QRS duration, as well as QT and QTc intervals. Sinus rate and sinus node recovery time were not influenced and refractory periods of right atrium, AV node, and right ventricle were unchanged. Like class I-C antiarrhythmic drugs, carocainide causes an increase of QRS duration without significant lengthening of the QT interval. In patients of Group II (n = 12), the ability of carocainide to interrupt supraventricular reentrant tachycardia was studied. In 11 of 12 patients (11 patients with regular atrioventricular reentrant tachycardia incorporating an accessory pathway, one patient with AV node reentry), sustained tachycardia was stopped by intravenous injection of carocainide in a dose of 3.5 mg/kg body weight. No major adverse effects were seen during or after administration of the drug. Carocainide was effective in the short-term treatment of supraventricular tachycardias. Further studies must show whether these promising effects can also be obtained during long-term oral treatment.