The present invention relates to a drug delivery system, comprising a reservoir from which set doses are apportioned, a dose setting mechanism by which the relative position of two elements is changed by a distance proportional to a set dose, and a push button by activation of which an element is moved a distance corresponding to the distance proportional to the set dose, further comprises a working cylinder having an open end into which a plunger fits, and a closed end at which an inlet and an outlet port connect the space beneath the plunger with an inlet communicating with the reservoir and an outlet communicating with a drug delivery member, respectively. Valves means at the inlet and outlet control the passages from the reservoir to the working cylinder and from the working cylinder to the drug delivery member, respectively. The cylinder with the plunger, and the injection member are independently exchangeable, disposable parts.