The efficacy and safety of intravenous propafenone was studied in 10 patients with Wolff-Parkinson-White syndrome and in 2 patients with a concealed accessory pathway. During electrophysiologic study, the effect of propafenone on the effective refractory period of the accessory pathway was determined, as well as its effect during orthodromic atrioventricular (AV) reentrant tachycardia and atrial fibrillation. Propafenone caused significant increases in the accessory pathway refractory period, both in the anterograde direction (290 +/- 19 versus 474 +/- 50 ms, p less than 0.05) and in the retrograde direction (238 +/- 15 versus 408 +/- 44 ms, p less than 0.05). Complete anterograde accessory pathway conduction block occurred in four patients. Sustained AV reentrant tachycardia was inducible in 11 patients before administration of propafenone. Drug infusion during AV reentrant tachycardia promptly terminated arrhythmia in 10 of these 11 patients and caused slowing of AV reentrant tachycardia in the remaining patient. Before propafenone, sustained atrial fibrillation was inducible in six patients and nonsustained atrial fibrillation in four patients. After propafenone, no patient had inducible sustained atrial fibrillation. Furthermore, propafenone caused a marked decrease in peak ventricular rate during atrial fibrillation. Eight patients have been treated with oral propafenone and followed up for 12 +/- 2 months. All have remained virtually free of recurrent arrhythmia and none has developed significant side effects. Propafenone is a very promising agent for emergency intravenous therapy as well as long-term oral therapy in patients with Wolff-Parkinson-White syndrome.