In this study we measured left ventricular function at rest and during exercise before and after operation in 18 patients who underwent mitral valve replacement (MVR) for chronic mitral insufficiency. First-transit radionuclide angiocardiography was employed a week before operation and 5 to 19 months after operation to calculate left ventricular ejection fraction, volumes, output, filling, and emptying rates. Preoperatively, left ventricular ejection fraction increased from 0.56 +/- 0.09 (mean +/- S.D.) at rest to 0.62 +/- 0.11 during exercise, and this response was not altered for the group after operation. After valve replacement, there was no consistent change in end-diastolic volume (EDV), and cardiac function was not related to an alteration in filling rates of the left ventricle. After operation, the patient group achieved a maximal ventricular filling rate during exercise comparable to that measured before operation. In addition, postoperative maximal filling rates did not correlate with prosthetic valve size. These data suggest that the hemodynamic changes elicited by MVR for chronic insufficiency are primarily related to the elimination of the regurgitant flow but no attributable to alterations of the myocardium. It is suggested that limitation in exercise performance after operation may be primarily related to the functional status of the myocardium prior to operation.