Fifty-six patients with atrial fibrillation associated with heart disease were treated with a modified electrode plate for low energy electric conversion. Fifty-four (96%) of these cases were successfully converted into sinus rhythm using the modified electrode plates, a new site and low energy discharge (an average of about 50 J). To construct the new-style electrode plates, a 2 cm arc segment was cut from each of two electrodes (10 cm in diameter each). As for the site of application, the cut-edge of the anterior chest electrode, which has a concave diameter of 150 cm, is placed close to the right of the sterum over the fourth to sixth intercostal spaces, and the cut-edge of the other electrode is placed to the left of the spinal column at the level of the seventh to ninth thoracic vertebrae. The safe-effective period for the appearance of the first post-discharge QRS complex was found to be 1.4-5.0 s.