A dispenser for receiving and dispensing volumes of fluid, on which a piston/cylinder unit having a piston and a cylinder, can be fitted in a releasable manner by means of a movement running at least substantially in an axial direction of the dispenser. The dispenser has a piston actuator for moving the piston relative to the cylinder. The piston actuator is arranged in a movable manner in the dispenser and is driven by means of a drive. The dispenser has a locking element which is arranged in a movable manner in the dispenser and is driven by a further drive. Also disclosed is a system for receiving and dispensing volumes of fluid, to a method for fitting a piston/cylinder unit in a releasable manner on a dispenser, and to a method for releasing a piston/cylinder unit which has been fitted on a dispenser.