The subject of this study is optimal mode control for the left ventricular bypass pump used to assist the failing heart. A mathematical model has been formulated for the overall cardiovascular system, both with and without bypass assistance. The closed-loop, uncontrolled model of the unassisted circulation has been implemented on an analog computer, and has been demonstrated capable of simulating normal as well as abnormal physiological behavior. Implementation of the bypass pump model has been shown to produce the general expected hemodynamic effects of decreasing pressure in the left atrium, while increasing that in the systemic arterial load. Four logical modes of pump control have been described and compared on the basis of their effect upon a scalar performance index used to reflect the basic goals of cardiac assistance.