An anaesthetic vaporizer includes a concentration dial for controlling the concentration of anaesthetic leaving the vaporizer. The vaporizer is adapted to be mounted on a back bar of an anaesthesia machine. An exclusion pin is mounted adjacent the back bar and forms part of an interlock system for preventing rotation of the concentration dial when a second vaporizer is also mounted on the back bar and is dispensing anaesthetic vapor. A button assembly is mounted on the concentration dial and must be depressed before the concentration dial can be rotated. The button assembly incorporates an overload device which, if excessive force is applied to the button assembly in order to override the effect of the exclusion pin will render the button assembly inoperative.