A peristaltic pump which may be used as an implantable medical device includes a rotor arm that is provided with a means for applying a regulated occluding force to the pump rollers. In a preferred embodiment, an expandable chamber containing a pressurized material may be incorporated into the pump. In this manner the pressure within the expandable chamber and thus the force applied to the pump roller may be kept constant despite local variations in the dimensions of the components and associated variations in the volume of the expandable chamber. Alternatively, the pressure within the expandable chamber may be modulated by the pressure of a reservoir communicating with the pump inlet or modulated by pump operating parameters such as motor torque.