The effects of tonic right and left vagal stimulation (RVS and LVS) on electrophysiological properties of the immature myocardium and specialized conduction system were evaluated in 11 neonatal canines pretreated with propranolol (1 mg/kg iv). Electrophysiological studies were performed by recording intracardiac electrograms from multiple endocardial catheters during programmed electrical stimulation. Assessments were made of sinus node function, intra-atrial, atrioventricular (AV) nodal and His-Purkinje conduction, and atrial and ventricular refractoriness in the control state and during RVS and LVS at 4-12 Hz. Vagal stimulation prolonged the sinus cycle length; RVS produced a 38% increase and LVS a 25% increase at 8 Hz (P less than 0.01). There were no changes in the intra-atrial or His-Purkinje conduction times. Comparable increases occurred during RVS and LVS in the paced cycle length resulting in AV nodal Wenckebach, the AV nodal conduction time at a paced cycle length of 340 ms, and the effective and functional refractory periods of the AV node, suggesting symmetrical influences of the right and left vagus on neonatal AV nodal function. Right atrial effective and functional refractory periods shortened significantly during vagal stimulation (ERP, 36% RVS and 23% LVS; FRP, 27% RVS and 15% LVS), and in 5 of 11 neonates, a sustained regular atrial tachyarrhythmia was induced during atrial extra-stimulation. Small yet significant increases were observed in the right ventricular ERP and FRP during vagal stimulation. This study provides information regarding the functional integrity of the parasympathetic nervous system and its potential role as a modulator of the electrophysiological properties of the newborn heart.(ABSTRACT TRUNCATED AT 250 WORDS)