Complete length-tension diagrams of myocardium were measured on isolated papillary muscles of cat right ventricle. These diagrams clearly demonstrate that irrespective of loading conditions there is no uniform maxima curve. As a function of preload the curves of afterloaded isotonic maxima obey totally separate nonlinear relations. Thus, for the heart muscle one should not assign a single linear regression line to the end-systolic length-tension points. Originally, its slope was supposed to be determined almost entirely by the contractile state of the heart muscle. Alteration in the contractile state of the heart muscle by varying the extracellular Ca++ concentration is primarily expressed in a corresponding change in the distance between maxima curve and resting length-tension curve, and thereby in the myocardial working capacity, whereas the mean slope of the maxima curves reacts in a considerably less sensitive manner. These evidences annul the supposed physiological basis of the end-systolic pressure-volume concept, with Emax as index of contractility.