Aim. To assess efficacy of a potassium channel blocker nibentan for pharmacological cardioversion of persistent atrial fibrillation. Method. Nibentan (1% solution) was infused intravenously after 3 weeks of warfarin pretreatment to 31 patients with duration of atrial fibrillation from 4 weeks to 15 months. Results. Sinus rhythm was restored in 24 patients. Mean dose of nibentan was 0.119±0.02 mg/kg. Proarrhythmic effects (paroxysmal or nonsustained polymorphic ventricular tachycardia) and substantial suppression of sinus rhythm automatism were registered in 3 and 3 patients, respectively. Conclusion. Nibentan is highly effective in persistent atrial fibrillation. It can be recommended for inhospital use in intensive care facilities.