To evaluate the effect of mitral valve replacement on left ventricular function in mitral regurgitation, we measured (1) the end-systolic stress/volume ratio, which is practically independent of changes in loading conditions, and (2) the left ventricular contractile reserves upon isometric exercise, both before and one year after mitral valve replacement in 11 patients with mitral regurgitation. The end-systolic stress/volume ratio increased, though the ejection fraction decreased after mitral valve replacement. The ejection fraction decreased and the end-systolic stress/volume ratio remained unchanged during isometric exercise before operation, whereas afterwards the ejection fraction remained unchanged and the end-systolic stress/volume ratio increased during exercise. Ventricular function improves after mitral valve replacement in patients with mitral regurgitation, though the ejection fraction, which is affected by altered loading conditions, deteriorates. The left ventricular response to stress caused by isometric exercise is also improved after surgery.