Of 16 infants who presented with paroxysmal supraventricular tachycardia in the neonatal period, 50 percent had an electrocardiographic pattern consistent with Wolff-Parkinson-White conduction, type A. It is suggested that infants have bypass pathways similar to or identical with a Kent pathway as part of normal maturation. Infants with paroxysmal supraventricular tachycardia have electrically active bypass tracts but these are documented in only about one half of the patients because of the short duration of recordings or because of concealment (the bypass tract conducts only in retrograde fashion). The activity of these pathways is enhanced by the predominant cholinergic innervation of the neonatal heart. Resolution of the arrhythmias and the Wolff-Parkinson-White pattern in most patients occurs because of anatomic maturation of the conduction tissue, development of adrenergic innervation and a decrease in cholinergic dominance. In some children, maturation is incomplete and the bypass fibers remain quiescent or become active under certain circumstances such as those associated with increased autonomic discharge. Extended surveillance is recommended for all infants who present with paroxysmal supraventricular tachycardia and the Wolff-Parkinson-White pattern.