The exercise electrocardiogram was studied in the patients with atrial fibrillation who were presumably free of hyperthyroidism, severe pulmonary disease or valvular disease. Thirty-one patients with atrial fibrillation, 25 male and 6 female, were subjected to a double Master's two-step exercise test. Seventeen out of thirty-one patients (54.8%) had abnormal electrocardiograms (positive test) according to our criteria after the double two-step test. Heart rate responding to the exercise increased much more than controls reported previously form our laboratory. It was assumed that in atrial fibrillation, an abnormally excess response of heart rate to exercise caused a marked rise in myocardial oxygen demands according to an increase in cardiac work and a decrease in effective coronary perfusion time. Furthermore, atrial fibrillation, probably absolutely irregular ventricular rate, might decrease coronary flow reserve responding to increase in oxygen demand. These suggest that abnormal hemodynamic responses to the exercise result in positive test even in the cases without coronary artery disease.