Twelve patients were reverted from atrial fibrillation to sinus rhythm by the technic of “cardioversion.” Brief, light anesthesia was the only drug employed. Under conditions of comparable oxygen consumption cardiac output was measured first in atrial fibrillation and then in sinus rhythm. Eleven patients were studied at rest and five during exercise.Ten of the 11 patients studied at rest showed a reduced arteriovenous oxygen difference with sinus rhythm and seven of the 11 increased cardiac output 0.6 L./min. or greater, an average increase of 34 per cent. All five patients studied at exercise decreased the arteriovenous oxygen difference with sinus rhythm and cardiac output rose 1.1 L./min. or greater in all, an average increase of 17 per cent.