The efficacy of intravenous propafenone (P) infused at 2 mg/kg in 3 min as a loading dose followed by 0.007 mg/kg/min along 24 hours, in converting atrial fibrillation (AF) was evaluated under continuous electrocardiographic and blood pressure control in 22 patients. In case of inefficacy after a wash out of 24 hours, amiodarone (A) 5 mg/kg in 3 min followed by 1.8 gr/24 hours was infused. AF had to be constant (at least one 24/hour Holter recording) and of recent onset. All the patients were NYHA class 1 or 2. Ten patients reverted to sinus rhythm after P usually within 60 min; two of the non responders reverted after A and ten did not revert at all. RR intervals were significantly shorter in the responders compared to the others: mean value 537 +/- 64 vs 771 +/- 200 msec (p less than 0.001). During P the QRS duration increased 12.9% (p less than 0.005) and QTc of a lesser extent 7.8%; during A QRS did not modify significantly and QTc prolonged 9.5% (p less than 0.002). Propafenone i.v. infusion appears to be a rapid effective method of converting recent onset AF to sinus rhythm in patients with high ventricular rate. Amiodarone i.v. does not significantly help in converting to sinus rhythm the patients non responder to P.