The efficacy and safety of oral nadolol in supraventricular tachycardia were evaluated prospectively in 27 children (median age 5.5 years). Fifteen patients had an unsuccessful trial of digoxin therapy. Intravenous nadolol was given to seven patients during electrophysiologic study; five of these had an excellent response and two had a partial response (25% decrease in tachycardia rate). Six of these patients had a similar response to oral nadolol. Twelve patients received both propranolol and nadolol. Among six patients, intravenous propranolol was successful in four and unsuccessful in two; all six had a similar response to oral nadolol. With oral propranolol, tachycardia was well controlled in four patients and persistent in two; five of five patients had a similar response to oral nadolol. Twenty-six patients were treated with oral nadolol; the arrhythmia was well controlled in 23, 2 had recurrent tachycardia and 1 patient had tachycardia at a 25% slower rate. The effective dose of nadolol ranged between 0.5 and 2.5 mg/kg body weight once daily (median dose 1 mg/kg per day). During follow-up (3 to 36 months), compliance and tolerance were excellent; excluding 2 patients with reactive airway disease who developed wheezing, only 3 (12%) of 24 had side effects necessitating a change in drug therapy. Once a day nadolol is a safe and effective agent in the management of supraventricular tachycardia in children. Its long-term efficacy can be predicted by the short-term response to intravenous nadolol or propranolol during programmed electrophysiologic study.