Propafenone was given to 60 children (mean age 4.5 years) with paroxysmal re-entrant supraventricular tachycardia (rSVT: 41 cases), postoperative automatic junctional tachycardia (JET: eight cases), automatic atrial tachycardia (AT:four cases), ventricular tachycardia (VT:four cases) and atrial flutter (AF: three cases). In acute cases (29) propafenone was administered intravenously (mean dose 1.3 +/- 0.5 mg.kg-1) chronic oral treatment (mean dose of either 11 +/- 3.3 mg.kg.day-1 or 265 +/- 78 mg.m-2 x day-1) was given to 48 children, in 2-3 divided doses. Overall efficacy was 76% for acute and 69% for chronic treatment, with best results in paroxysmal rSVT. It was effective in 89% acutely of those treated acutely and in 69% of those administered chronically. The efficacy of propafenone in the prevention of rSVT was strictly related to the incidence of attacks before treatment:propafenone was indeed inefficacious in 6/11, 6/14 and 0/12 of patients with daily, weekly and monthly attacks respectively. Side effects were observed in 25% of patients: 6% required suspension of therapy due to pro-arrhythmic effects in one patient on intravenous administration, peripheral neuropathy in one case and hypotension in two postoperative JET patients. In conclusion, its efficacy and limited side effects make propafenone a relatively safe and very useful drug in the treatment of various tachyarrhythmias. However, risk of unpredictable toxic levels requires careful use of the drug especially in the first days of treatment, when a daily monitoring of ECG is strongly recommended.