Elective direct-current countershock was applied to 65 patients with chronic atrial fibrillation to define the most exact criteria for safe use of this method. The 35 patients of group 1 were selected because of uncontrollable ventricular rate, persistent congestive heart failure, or a disabling awareness of the irregular rhythm. Sinus rhythm was reestablished in 31 patients (88%), but was maintained for two months in only ten (32%). Conversion following open heart surgery for mitral stenosis was successful in 28 of 30 (93%) patients; only three (10%) remained in sinus rhythm for more than two weeks. Fatal ventricular fibrillation occurred 30 to 120 seconds following shock for paroxysmal atrial tachycardia with block in two patients and intermittent ventricular tachycardia in one. Digitalis intoxication was implied and must be identified as a potential hazard.