An investigation was carried out in dogs to determine how the acceleration of blood in the aorta (dV/dt), as a new index of myocardial contractility compared with existing indices and how they correlated with each other. It was found that the indices derived from velocity and flow of blood, dV/dt max and dQ/dt max, and from intraventricular pressure, dP/dt max, correlate well with each other but there is less agreement between them and the reciprocal of the Pre-Ejection Period, l/PEP. The ratio dP/dt max divided by I.P. correlated well with dQ/dt max divided by I.Q. and dQ/dt max divided by Q. max but not so well with dV/dt max divided by I.V. (instantaneous velocity) of dV/dt max divided by V. max and the ratios dQ/DT DIVIDED BY I.Q. and dQ/dt divided by Q max as well as l/PEP. In view of the lack of agreement of quantitative definition of myocardial contractility, the ratio dV/dt divided by V. max would have several practical advantages as an indicator of the inotropic state of the heart; these are that the probe used to establish descending aortic blood velocity does not require calibration, and the signal can be obtained by a relatively non-invasive technique that is suitable for patient care and yet agrees with other established indices of myocardial contractility.