Prolonged atrioventricular (A-V) conduction is often an apparently normal finding in children with or without congenital heart disease. The mechanism is probably vagally mediated and appears to be benign in the majority of cases. Eleven children with first-degree and/or second-degree Mobitz type 1 A-V block were studied by intracardiac His bundle electrocardiography with atrial pacing. Six children had congenital heart disease with left-to-right shunts and 5 had normal cardiac anatomy. In all the patients studied, the A-H intervals were longer than normal. Wenckebach periodicity appeared at relatively long cycle lengths, and in 5 of the 6 patients in whom values could be determined, atrioventriculonodal effective and functional refractory periods were prolonged. With atropine and isoproterenol conduction intervals returned to normal. There was no tendency to develop echo beats with atrial extrastimuli, even when A-H intervals became markedly prolonged. The results of this study suggest that vagal tone plays a significant role in the prolongation of A-V conduction, even in congenital heart lesions that are known to be associated with a long PR interval.