We have tested the validity of the following indices of myocardial contractility: the maximum acceleration of blood from the left ventricle, the maximum rate of rise of left ventricular pressure (LV dP/dt max), LV dP/dt max/left ventricular pressure at the time of LV dP/dt max (LV dP/dt max)/P, the maximum of the instantaneous ratio of the rate of rise of left ventricular pressure [(LV dP/dt/P) max], and the X-Y plot of LV dP/dt/P against P. In the isolated cat heart, the filling and resistance to ejection of the left ventricle were controlled directly. The other preparations were: the open chest anaesthetized dog, the closed chest anaesthetized dog with and without denervated heart, and the conscious dog with and without denervated heart. In these preparations, the effect of increased load was studied by constricting the aorta between beats and the effect of increased heart size was studied in subsequent beats during ventricular distention. It was found that none of the indices was insensitive to increased load and heart size under all circumstances. We were unable to find a satisfactory method of extrapolating the plot of LV dP/dt/P against P to zero P to obtain Vmax. [f a linear relationship was assumed, P was found to be a complex function of time (t), namely Po/[l+(Po/Pd-1)e-Vmaxt] where Po = P when dP/dt/P=0 and Pd = end-diastolic pressure.