Adenosine has become the drug of choice for termination of regular, normal QRS tachycardia. Initial studies in adult and pediatric patients have shown that the drug is effective for tachycardias using the atrioventricular (AV) node as an integral part of the tachycardia circuit and has few serious side effects. Experience with adenosine administration in children was reviewed to examine the diagnostic and therapeutic usefulness, effective dose, and adverse effects of adenosine. Adenosine was administered to 38 children during 50 separate electrophysiologic evaluations. Eleven patients had structural or acquired heart disease. Tachycardia mechanisms included orthodromic-reciprocating tachycardia using an accessory AV connection (23 patients), primary atrial tachycardia (6 patients), AV node reentrant tachycardia (3 patients), ventricular tachycardia (2 patients), postoperative junctional tachycardia (1 patient), and antidromic-reciprocating tachycardia (1 patient). Adenosine successfully terminated 51 of 53 episodes (96%) of tachycardia using the AV node, 5 of 10 primary atrial tachycardias, 1 of 1 junctional tachycardia, and 1 of 3 ventricular tachycardias. Reinitiation of tachycardia was seen after 16 of 58 successful terminations (28%), reducing the effectiveness to 39 of 53 (74%) for tachycardia requiring the AV node. Average effective dose was 132 μg/kg, range 50 to 250 μg/kg, and was slightly higher for peripheral (147 μg/kg) than for central (120 μg/kg) administration. Significant complications occurred in 4 of 38 patients, including atrial fibrillation, accelerated ventricular tachycardia, apnea, and 1 minute of asystole. Although adenosine is useful therapeutically and diagnostically in children with tachycardia, its effectiveness is limited by tachycardia reinitiation and adverse effects. Higher doses may be required for peripheral intravenous administration.