ELECTRICAL countershock has been found to be a useful method of terminating ventricular fibrillation since the end of the last century.1 However, there are still aspects related to this procedure that are not well known,2 especially in reference to the prevention of recurrence of the original arrhythmia. AC current was originally used by Zoll and co-workers3 primarily for the emergency treatment of ectopic ventricular rhythms.4,5 More recently, DC current has been employed successfully by Lown et al not only for emergency ventricular fibrillation but also for abolishing susupra-ventricular arrhythmias.6,8 Several other workers have performed elective defibrillation of the atria.9,13 In a previous report from our department dealing with problems related to conversion of atrial fibrillation,2 it was observed that a significant number of patients reverted to the original arrhythmia shortly after sinus rhythm was established. The number of recurrences of atrial fibrillation was decreased by premedication with quinidine sulfate.