Actual researches show that adenosine and its forerunner, ATP, can realise a nodal block in intravenous (i.v.) administration. This effect, even if transient, is of particular usefulness in the therapy of paroxysmal supraventricular tachycardia (PST) in children and adults. 1 mg/kg ATP was administered i.v. to 51 children (aged 3 months to 15 years) admitted to the Intensive Care Unit for severe regular paroxysmal tachyarrhythmias with clinical and ECG symptoms. In 31 cases, the P wave was absent on ECG. In 6 of 31 cases in whom wide QRS complexes were found (over 0.10 sec), a WPW syndrome (4 cases) or bundle branch block (2 cases) was associated. Rapidly i.v. administration monitored electrocardioscopically was followed by ending of PST in 43 children, including those with wide QRS complexes. The highest effectiveness was in junctional tachycardia by re-entering mechanism. The drug was well tolerated, had no side effects or hemodynamic disturbances and this is very important, because in 12 infants already existed symptoms of congestive cardiac failure at their admission. The use of i.v. digoxin with/without propranolol was necessary in 8 cases, in which the repeated ATP administration was ineffective. We consider that i.v. ATP administration in regular tachyarrhythmias in children is very useful for the proper diagnosis of the types with wide QRS complexes and/or when the P wave did not appear on the ECG, and also for the treatment, because ATP has a high effectiveness and an excellent tolerance.