Fourteen patients with established atrial fibrillation (longer than three months) that was refractory to treatment were studied to compare the clinical and electrophysiological effects of amiodarone and bepridil. All patients initially received bepridil for three weeks (200-600 mg/day), followed by amiodarone for two to three months (100-400 mg/day). Bepridil seemed to be slightly more effective than amiodarone in converting the fibrillation to sinus rhythm (nine of fourteen compared with four of ten). The ventricular response in atrial fibrillation was equally well controlled by bepridil and amiodarone, both at rest and during exercise. Bepridil was associated with the development of ventricular arrhythmias in eight of fourteen patients; two had torsade de pointes, which in one degenerated into fatal ventricular fibrillation. These arrhythmias seemed to be associated with bepridil induced prolongation of the QTc interval. No ventricular arrhythmias were seen during amiodarone treatment. Although bepridil seems to be an effective antiarrhythmic agent for the management of atrial fibrillation, its arrhythmogenic actions make it unsuitable for this purpose.