Adenosine was administered to 25 infants and children (11 patients after presenting with a sustained arrhythmia, and 14 during a diagnostic electrophysiologic study) to determine its electrophysiologic effects. Adenosine was given as an intravenous bolus (starting dose 37.5 micrograms/kg, and increased by 37.5 micrograms/kg increments until an effect was seen). Adenosine caused tachycardia termination or transient increased atrioventricular (AV) block in all 25 patients. Seven patients had tachycardia requiring only the atria for perpetuation and developed increased AV nodal block (minimum effective adenosine dose range 37.5 to 350 micrograms/kg, mean 131). Thirteen had AV reciprocating tachycardia or AV node reentry tachycardia (minimum effective adenosine dose range 37.5 to 225 micrograms/kg, mean 114). Four other patients received adenosine to rule out preexcitation (minimum effective adenosine dose range 37.5 to 375 micrograms/kg, mean 165). One of the 25 patients had junctional ectopic tachycardia and adenosine administration caused retrograde AV block. Six of the 25 (24%) had noticeable but minor side effects. One patient had sustained bradycardia (2 to 3 minutes requiring temporary pacing). Adenosine is a safe and effective agent in the evaluation and treatment of infants and children with arrhythmias.