Abstract: The invention relates to an apparatus for releasing a predetermined amount of a free-flowing medium into a fluid container, in particular the flushing cistern of a water closet. The apparatus comprises a pump, in particular diaphragm pump, which comprises an actuating plunger (6) that cooperates with a diaphragm (5) and is movable upward and downward by means of a float (7) as a result of change in the water level within the fluid container (20). The apparatus further comprises a reservoir (2) for the free-flowing medium, which is coupled to the input of the diaphragm pump, and a nonreturn valve (9) associated with an output (22) of the diaphragm pump. It is provided in accordance with the invention that the diaphragm (5) has a sacklike form and that the actuating plunger (6) acts at the end of the sack in such a way that during such action the diaphragm sack (5) is indented so as to enclose the contact surface (6a) of the plunger (6).