Using intracardiac electrogram recording and stimulation techniques, 13 neonatal puppies (3-15 days) and 9 adult dogs were studied following cumulative doses of intravenous propranolol. Although propranolol caused a dose-dependent increase in the sinus cycle length in both groups, the increase was significantly less in the neonates than in the adults. In both groups the pacing interval resulting in atrioventricular (AV) nodal Wenckebach was increased significantly by propranolol; however, the increase was again less in the neonates than in the adults. There were comparable dose-dependent increases in atrial and ventricular refractoriness in both neonate and adult groups.