43 patients, 3 days to 12 years old, presented 68 paroxysms of supraventricular tachycardia and 2 crises of atrial flutter. 6 patients had a WPW syndrome and only 7 had an associated cardiac disease; the remaining had no other complaint but the arrhythmia. Verapamil, administered in the dose of 0.125-0.25 mg/Kg intravenously, interrupted critically the tachycardia in virtually all cases. The flutter was converted to atrial fibrillation with a slower ventricular rate. No adverse effects were observed but in two cases: one had a cardiac arrest promptly relieved and another a ventricular bradycardia corrected by atropine. Both cases had received an overdose of the drugs.