The efficacy of propafenone in the treatment of paroxysmal atrial fibrillation was investigated in 16 patients presenting with frequent and/or poorly tolerated paroxysmal atrial fibrillation despite prophylactic treatment in 13 patients. Prolonged atrial fibrillation (> 10 minutes) was triggered in 16 patients following a standard atrial stimulation protocol. Intravenous injection of propafenone (2 mg/kg over 5 minutes) restored sinus rhythm in 12 patients (75%) within 15 minutes after the end of the injection. In five of the 10 patients in whom this was possible, propafenone prevented the induction of atrial fibrillation in response to programmed stimulation by the same protocol. Oral propafenone (900 mg/24 hours) was indicated in 11 patients. The treatment was discontinued in 1 patient due to severe dizziness. Atrial fibrillation recurred in 4 patients 10 to 91 days after treatment began. With a mean follow-up time of 8 months, 4 patients had been brought under control and 2 had relapsed despite a generally beneficial effect. Three patients suffered side effects which did not make it necessary to stop treatment. This study suggests that intravenous propafenone is effective against persistent atrial fibrillation induced by stimulation. Oral propafenone may be useful for the prevention of attacks of recalcitrant paroxysmal atrial fibrillation.