One hundred and ninety-six patients with stable atrial fibrillation of recent onset received propafenone (98 patients) or Amiodarone (98 patients) intravenously. Eighty-nine subjects (90.8%) who received propafenone, and 79 patients (80.6%) who received amiodarone were converted to sinus rhythm (p > 0.05). The mean conversion time was 2.51 +/- 2.77 hours after propafenone and 11.21 +/- 4.32 hours after amiodarone (p < 0.0005). The same drug was continued per os in converted patients, and efficacy in maintaining sinus rhythm was evaluated in a one-year follow-up. Symptomatic atrial tachydysrhythmias were observed in 20 (22.4%) of the 89 patients treated with oral propafenone and in 12 (15.18%) of the 79 patients receiving oral amiodarone (p > 0.05). Conversion to sinus rhythm was affected by the duration of arrhythmia and by left atrial size, the latter determining sinus rhythm stability. Both drugs used intravenously are efficacious and safe. Propafenone showed a more rapid action. Oral treatment with low doses for long periods of time with both drugs showed neither important side effects nor proarrhythmic consequences, with satisfactory results.