The invention relates to a method for controlling an assist pump for fluid delivery systems with pulsatile pressure. If, for example, a heart-assist pump is operated with a constant rotational speed, the blood is continually delivered through the assist pump even when the cardiac chamber is in the decontraction phase. The inventive method ensures that an assist pump functions in an assisting manner only during the pressure phase of the main pump by constantly determining the pressure difference between the inlet and delivery side of the assist pump and by constantly determining the rate of flow through the assist pump. The rotational speed of the assist pump is controlled as to prevent the determined pressure difference from falling below a predeterminable value and to prevent the rate of flow from falling below zero.