Ninety-two patients were entered in a double-blind controlled trial of long-acting quinidine bisulphate for the maintenance of sinus rhythm following DC reversion of atrial fibrillation. By random selection two statistically comparable groups of patients were obtained, a control group being given a placebo and a treated group quinidine bisulphate 1·5 g. per day in two 12-hourly doses. Thirty-seven patients were followed up in the control group and twenty-eight patients in the treated group. The maintenance of sinus rhythm in the treated group was significantly longer than in the control group, especially during the first three months. The mean serum quinidine level in the treated group was 2·2±0·3 (SEM) mg. per litre. There was one death among the patients treated with quinidine, which may have been due to quinidine toxicity caused by a high serum quinidine level.