Efficacy and safety of intravenous flecainide (2 mg/kg body weight in 10 minutes), verapamil (10 mg in 1 minute), and propafenone (2 mg/kg body weight in 10 minutes) were investigated in 90 consecutive patients with atrial fibrillation (AF) or flutter (AFL). In the first 40 patients, flecainide and verapamil were evaluated; in the second 50 patients, flecainide and propafenone were compared, both in a single-blind randomized study design. The primary end point was sinus rhythm occurring within 1 hour after start of infusion. Sinus rhythm was attained in 32 of 37 patients (86%) with AF treated with flecainide and in 11 of 20 patients (55%) with AF treated with propafenone. In recent onset AF (less than or equal to 24 hours) conversion rates were 24 of 25 patients (96%) in the flecainide group and 8 of 14 patients (57%) in the propafenone group (p less than 0.05). Conversion of AFL occurred in only 1 of 8 patients (13%) in the flecainide-treated patients and in 2 of 5 patients (40%) treated with propafenone (difference not significant). Verapamil was almost ineffective, since only 1 of 20 patients (5%) responded within 1 hour. Time to conversion was 21 +/- 17 minutes in the flecainide group and 16 +/- 10 minutes in the propafenone group. QRS widening occurred in flecainide-treated patients (83 +/- 15 to 99 +/- 20 msec; p less than 0.001), but not after propafenone (83 +/- 11 to 86 +/- 12 msec). Significantly higher plasma levels were found in patients with conversion within 1 hour using propafenone.(ABSTRACT TRUNCATED AT 250 WORDS)