Abstract: The present invention is a liquid pumping system with constant flow rate. The system contains at least one first cylinder means and reciprocating piston means defining a first pumping capacity (V.sub.1), at least one second pumping unit including cylinder means and reciprocating piston means defining a second pumping capacity (V.sub.2) and at least one complimentary pumping unit including cylinder means and reciprocating piston means defining a complimentary pumping capacity (V.sub.0) the driving means for actuating the piston means synchronously but in phase opposition between the piston means of the first unit and the second and complimentary pumping units in such a manner that each piston performs alternating suction and discharge strokes linearly having feed lines and check valves wherein V.sub.1 equals V.sub.2 plus V.sub.0.