Six patients including three females with WPW syndrome (three with type A and three with type B) presenting with recurrent paroxysmal tachyarrhythmia were subject to electrophysiological studies. Apart from basal parameters, rapid atrial and ventricular pacing was done which confirmed electrophysiological characteristics of bundle of Kent operation. A single intravenous bolus of ajmaline 50 mg. was effective in blocking the bundle of Kent in all patients within 30 seconds of injection with the effect persisting for 15 minutes in one and from 25 to 60 minutes in the others. The most dramatic effect was prolongation of the HV interval with normalization of QRS complex with a marginal effect on the AH interval. The drug was also effective in breaking narrow QRS tachycardia in two patients and broad QRS tachycardia in one patient. Long-term oral therapy with ajmaline has proved effective in preventing recurrent tachyarrhythmias. We conclude that ajmaline is specifically effective and safe in the treatment of the WPW syndrome.