Abstract: An apparatus serves for preparing a mixture of an active agent and a diluting agent. A container (45) serves for receiving the diluting agent. A cartridge (10) serves for receiving the active agent (41) within an interior (40) of the cartridge (10) being delimited by wall sections (12, 14, 34). The cartridge (10) is adapted to be inserted into the container (45). It is configured such that the active agent (41) may be discharged into the container (45) and diluted therein with the diluting agent. The cartridge (10), further, has a first wall section (34) adapted to be at least partially disconnected from a surrounding second wall section (14). The first wall section (34) is, further, connected to a third wall section (12) via an actuator (21), the third wall section (12) being separated from the first wall section (34) by the second wall section (14). The actuator (21) is rigidly connected to the third wall section (12) at a fastening point.