Thirty-three patients with Wolff-Parkinson-White syndrome were studied electrophysiologically before and after administration of intravenous procainamide and oral quinidine sulfate. Procainamide prolonged the shortest R-R (SRR) interval between two consecutive pre-excited beats during atrial fibrillation 20-70 msec in 15 of 21 patients with no change observed in 6 of 21 patients. Quinidine sulfate prolonged the SRR 20-170 msec in all 16. In 14 of 18 patients where procainamide and quinidine were comparable, quinidine prolonged the SRR 30-100 msec more than procainamide.