Optimal value of stroke volume ejected against a given arterial load is studied, i.e. left-ventricular function is described by an optimization model. In the model, end-diastolic volume (Ved), the linear end-systolic pressure-volume relation, heart rate, and ejection time are given, and arterial load is described by the three-element windkessel model. The cost function of the model takes into account two optimality criteria for ventricular function: energy economy and efficient response to an increase in Ved. The observed stroke volumes of isolated canine hearts could be predicted quite accurately by the model. It is concluded that the left-ventricular response to a change in arterial load and Ved can be explained by an optimization model when contractility and heart rate are kept constant. The results also strongly suggest that energy economy and efficiency are essential features of left-ventricular function.