Five patients, age 3 weeks to 11 years, presented with supraventricular tachycardia that remained uncontrolled following adequate digitalization. Four of these patients underwent invasive electrophysiologic studies to determine the mechanism of the arrhythmias. Of these four patients, three had concealed Wolff-Parkinson-White syndrome, and one patient had evidence of dual A-V nodal pathways. Propranolol was added to the medical treatment and was administered orally in doses ranging from 7 to 14 mg/kg/day (average 9 mg/kg/day). All five children remain free of their tachycardia except for one patient who occasionally has supraventricular tachycardia with febrile illnesses. No adverse reactions to these high doses of propranolol were encountered.