Direct current countershock was given to 46 patients with chronic atrial fibrillation of diverse causes, with successful conversion in 87 per cent of them. Patients were observed while receiving quinidine prophylaxis, with serial determination of serum quinidine levels, for up to 22 months. Two quinidine preparations were employed, one a long-acting form, in various dosage regimens and both preparations were found to be effective prophylactic agents for maintaining reliable serum quinidine levels. Fifty per cent of patients remained in normal sinus rhythm at three months, 28 per cent at six months and 13 per cent at twelve months.