Abstract: A positive displacement pump having a variable flow volume comprises a housing having an outer ring inserted in an internal space of the housing, the outer ring supporting a rotor set comprising a rotor, rotor elements and a cam ring. The rotor is surrounded by the cam ring. Working chambers are formed between the rotor and the cam ring, the volumes of the chambers being adjustable by varying the eccentricity between the rotor and the cam ring. Pressure chambers are formed between an outer peripheral surface of the cam ring and the outer ring. A control piston is connected to the pressure chambers by way of associated control bores which extend at least approximately axially in the cover plate radially inside a peripheral flange of the housing and extend through a face plate which is disposed on the inside axially in front of the rotor set into the pressure chambers.