In canine heart-lung preparations the relation between dP/Dt max and end-diastolic pressure exhibits the steepest slope shortly after isolation of the heart. When the heart is deteriorating, the slopes of the preload dependence of right and left ventricular dP/dt max are progressively reduced, but increase again after application of calcium. The preload dependence of dP/dt max is influenced by the contractile state of the myocardium. In the right and left ventricle the change in preload dependence of dp/dt max show a linear correlation. In the individual heart the slopes of the preload dependence of dP/dt max and of aortic flow (representing ventricular function curves) change in a very similar manner with varying contractility. In 33 different isolated hearts the slopes of the preload dependence of the two parameters show a good correlation shortly after isolation of the hearts. The preload dependence of dP/dt max can not only be used for assessment of acute variations in in myocardial contractility, but appears also to be at least as reliable as the ventricular function curves for the evaluation of the basic contractile state of different hearts. This study further shows that if dP/dt max is used for evaluation of relative changes in contractility, the quantitative results are influenced by the end-diastolic pressure level chosen.