Propafenone, an antiarrhythmic drug of IC type, was applied to 10 patients with supraventricular tachycardia (SVT) produced by intranodal reentry (group I) and in 14 patients with reentry by an accessory atrioventricular (AV) pathway (group II), 10 of them suffering from orthodromic SVT. Propafenone given intravenously depresses or blocks the antegrade or retrograde conduction in the AV node and in the accessory AV pathway. The same effect is observed with orally given propafenone: 66% of antegrade blocking and 54% of retrograde blocking of the accessory conduction pathway. Intravenously given propafenone reduces within 2 to 3 min by antegrade or retrograde blocking 70% of SVT produced by intranodal reentry and by 85% of SVT produced by reentry by the accessory pathway. After injection it becomes impossible to induce intranodal SVT in 60% of cases and SVT by the accessory pathway reentry in 28% of cases. With oral treatment (600 mg/day) reinduction of intranodal SVT becomes impossible in 66% of cases and of SVT produced by reentry by the accessory pathway in 42% of cases. Long-term oral administration (17 +/- 3.7 months) of the same dose prevents 88% of SVT produced by internodal reentry and 80% of spontaneous SVT produced by reentry by the accessory pathway. Cardiologic tolerance is satisfactory: one case of atrioventricular and intraventricular dysrhythmia is observed. The same holds true for general tolerance: in 2 cases drug administration is discontinued and 11 patients present neurologic and digestive troubles improving after lowering the dosage or increasing the fractionation.(ABSTRACT TRUNCATED AT 250 WORDS)