Right and left heart hemodynamics were studied in 12 patients with mitral valvular disease and atrial fibrillation before and immediately after external direct-current countershock resulting in restoration of normal sinus rhythm. The change of rhythm was accompanied by the following hemodynamic variations in the majority of cases: Systolic and mean pressures in the right and left ventricles and in the pulmonary and brachial arteries were not systematically altered, with a tendency, however, to a decrease in ejection pressures in the right heart and increase in the left heart. Mean right atrial and right ventricular end-diastolic pressures as well as left ventricular end-diastolic pressure decreased significantly. The behavior of the mean left atrial pressure appeared to be influenced by the degree of mitral valvular disease. Cardiac output and index and stroke volume and index increased; pulmonary vascular and systemic resistances decreased.The stroke work of the ventricles also increased in the majority of patients.An analysis of the changes in left ventricular end-diastolic pressure in relation to the left ventricular stroke work indicated that sinus rhythm resulted usually in a shift of the ventricularfunction curve to an area of improved performance.