A method is reported by which the changing capacity factor of the tension-time values of the ventricular pressure curves is taken into account. It was applied to intraventricular pressure curves recorded from intact animals under a variety of circulatory conditions. The area of the pressure curve is subdivided into 4 parts, A, B, C, D. by vertical lines at the onset and end of ejection and a horizontal line at the diastolic pressure level. The changes in the isometric areas A and C are described and incidentally discussed. Attention is directed to the ejection areas B and D. Area B, the lower area, is regarded as a measure of the static effort of the ventricle during ejection in the sense that it represents the pressure energy required to sustain the blood column at a diastolic level. The upper area, D, is regarded as an index of the dynamic effort during ejection in the sense that this energy becomes available for conversion into kinetic energy of flow at some subsequent period of the cardiac cycle. A comparison of the 2 areas (as a ratio D:B) gives an estimate of the economy with which the pressure energy is utilized during ejection, as the changes in ventricular volume affect both areas simultaneously and equally during successive moments of ejection. When the initial dis-tention is increased, without altering the arterial resistance or heart rate, by an augmented venous return, the combined areas of B and D increase. This is due to a prolongation of ejection, a greater amplitude and contour changes of the summit. Area D increases more than B, indicating a more efficient utilization of the extra energy developed during ejection. An increased arterial resistance, with the other 2 factors constant, decreases the combined areas of B and D and the efficiency of the energy utilization during ejection; it is thus without stimulating action as far as economizing the energy of the heart beat. In the intact animal, the residual blood, by distending the ventricle, counteracts this detrimental effect of the increased arterial resistance. This compensatory increase in initial volume acts as "a factor of safety." The effect of initial volume and arterial resistance complement each other when the heart rate changes. For example, at slow rates the initial volume increases and the diastolic pressure level falls, both factors tending to increase the combined area D and B and the ratio of area D to area B. These observations explain why the slowly beating heart is spared and the rapidly beating one more prone to fatigue.