The electrophysiologic effects of intravenous (i.v.) flecainide were evaluated in 13 patients (pts) with recurrent paroxysmal supraventricular tachycardia (PSVT): 6 pts had an overt accessory pathway, 2 a concealed anomalous pathway and 5 had an idionodal reentrant tachycardia (AVNRT). Another patients with overt preexcitation underwent electrophysiologic testing as part of a diagnostic investigation for syncope. After flecainide the effective refractory period of the right atrium and retrograde AV node, and anterograde and retrograde Wenckebach point significantly increased. The drug blocked retrograde conduction on the accessory pathway in 3 pts whereas anterograde conduction was blocked in all 7 pts with overt anomalous pathway. The mean cycle length of the atrioventricular reentrant tachycardia (AVRT) and of the AVNRT increased respectively from 269 +/- 34 msec to 332 +/- 25 msec (P less than .005) and from 286 +/- 9 msec to 380 +/- 64 msec (P less than .05). After i.v. flecainide, reentrant supraventricular tachycardia was no longer inducible in pts with AVRT and 1 with AVNRT, inducible but non sustained (less than or equal to 30 seconds in duration) in 1 pt with AVRT and in 3 with AVNRT. Thirteen pts continued oral flecainide treatment for a mean of 7.2 +/- 3.6 months (range 3 to 12 months). Tachycardia recurred in all 3 pts whose arrhythmia remained inducible and sustained after i.v. flecainide, and in 1 of 10 pts whose re-entrant supraventricular tachycardia was suppressed (6 pts) or inducible but non sustained (4 pts). Thus flecainide is an highly effective and well tolerated drug for the control of PSVT in infancy. The electrophysiologic drug testing with flecainide predicts its efficacy during chronic therapy in most patients.