The diagnostic and therapeutic utility of adenosine triphosphate (ATP) in pediatric age was investigated in fifteen children aged 4 days-16 years (mean age 6.4 years) observed for paroxysmal (Group A-9 pts) or incessant (Group B-6 pts) tachycardia. Twelve patients underwent transesophageal electrophysiological study. ATP was given as an intravenous bolus (0.075-0.5 mg/Kg). In Group A patients, ATP resulted in termination of spontaneous or induced tachycardia, and in all cases interruption in anterograde limb of the re-entry circuit occurred. In Group B patients, ATP induced transient atrioventricular block with persistence of atrial tachycardia, suggesting the atrial origin of the arrhythmia. No adverse haemodynamic effects were observed in any patient. We conclude that in pediatric age ATP must be considered the drug of first choice for junctional reciprocating tachycardias because of its efficacy, short mid-life and insignificant side-effects. Furthermore, it represents an effective diagnostic test for differentiating between junctional reciprocating tachycardias and atrial ectopic tachycardias.