Abstract: Device for mixing a pharmaceutical composition stored in a chamber (44) in said device with an agent to be added shortly before administration to form a preparation to be adminstered. The device should be capable of being shaken without leaking. It comprises a hollow body (2) having an outlet (4) sealed by a removable closure (12) and a plunger (6), which is slidable therein in sealing contact with the inner wall of the hollow body (2). The plunger (6), the hollow body (2) and the removable closure (12) define said chamber (44). An actuating means (22) is connected to the plunger (6) for displacing the same in the hollow body (2). A filling conduit (19) for said agent is connected to the chamber (44). A check valve (30) is associated with the conduit (19) and the chamber (44), which prevents flow from the chamber (44) but permits flow into said chamber through said conduit (19).