Sotalol is a beta-blocker with class III antiarrhythmic properties that has recently been used in children for the treatment of supraventricular and ventricular arrhythmias. However, little is known about its electrophysiologic effects on the immature heart. Using intracardiac electrocardiographic recordings and stimulation techniques, 15 canine neonates (8-15 days) and 15 adult mongrel dogs were studied with cumulative doses of sotalol (0.5, 1, 2, and 4 mg/kg plus an additional dose of 8 mg/kg for neonates). Heart rate decreased significantly in the two groups, but more in adult dogs (-43% in adult dogs versus -25% in neonates, p less than 0.05). There was no significant change for QRS duration and His-Purkinje system conduction time interval. QT and atrioventricular nodal conduction time intervals increased in adult dogs and neonates. Sinus node recovery time increased significantly in the two groups, but more in adult dogs. Refractory periods of the atrioventricular (AV) node increased significantly in neonates. Atrial flutter was no longer inducible in 12 of 15 neonates after the 2 mg/kg dose. Atrial effective refractory period increased significantly more in neonates (96%, p less than 0.001) than in adult dogs (58%, p less than 0.001). Ventricular effective refractory periods increased significantly both in neonates (46%) and adult dogs (50%), in a similar way. In conclusion, sotalol has greater electrophysiologic effects on the immature heart at the atrial level when compared to the adult, and similar effects on the refractory period of AV node and ventricle.