Cardiac output was determined by the indicator-dilution technic with injection of indicator into the ascending aorta, left ventricle, and pulmonary artery in 13 cases of mitral incompetence and in 15 control cases (normal or mild valvular obstruction). The ascending aortic dye curve is not involved in mitral incompetence in its primary circulation and was used for comparison.In normal subjects, there is no significant difference between the cardiac output obtained by injection of indicator into the ascending aorta and the left ventricle.The cardiac output obtained from indicator-dilution curves affected by mitral regurgitation in its primary circulation is underestimated by approximately 20 per cent.The effects of the coronary recirculated indicator particles on the disappearance slope would best explain the discrepancy. Even in the normal circulation, the inclusion of the coronary recirculated particles leads to a small underestimation of the cardiac output when the indicator is injected into the pulmonary artery.