1. In normal, conscious dogs, I.V. infusion of isoprenaline caused inases in heart rate and the maximal derivatives of left ventricular pressure (dP/dt max) and left ventricular internal diameter (dD/dt max). The changes in both derivatives were linearly related to the increase in heart rate.2. Increments in heart rate produced by right atrial pacing caused only minimally significant increases in both dP/dt max and dD/dt max at heart rates of 180 beats/min. Increases in heart rate, with end diastolic diameter maintained constant, resulted in small but significant increases in dP/dt max and no significant increase in dD/dt max.3. Increasing preload by volume infusion had little effect on either derivative, while increasing afterload by phenylephrine administration produced a small but significant increase in dP/dt max and no change in dD/dt max.4. Both dP/dt max and dD/dt max are equally reliable as indices of the inotropic state of the myocardium and are minimally influenced by changes in preload, afterload or heart rate.