The invention relates to a assembly for administering to a patient a dose of active material, e.g. a radio active substance containing fluid and/or a contrast medium fluid for imaging purposes. The assembly comprises a container (3), having a ejection side (6) with an exit and a control side and a plunger movable from the proximal to the distal side of the container to eject the dose, which is pre-filled in the container, from the exit. Further, the assembly comprises a conduit configuration, which is connectable to the exit of the container and to the patient. The plunger (4) in the assembly comprises a passage for a flushing fluid, which passage is normally closed and selectively opened. For selectively opening the passage the assembly comprises an opener (17), which is arrangable at the exit to act on the plunger to open the passage with the plunger moved to a position in proximity of the exit.
