In a first aspect, the invention relates to an apparatus for inducing or maintaining a target end tidal concentration of a gas in a subject comprising a breathing circuit, a source of gas flow into the circuit, means for controlling the rate of the source of gas flow into the circuit and means for controlling the concentration of gases in the source gas flow independently from each other. In another aspect, the invention relates to a method of preparing an apparatus for inducing or maintaining a target end tidal concentration of a gas X in a subject comprising selecting a rate of a source gas flow into a breathing circuit, selecting the concentration of at least one constituent gas of a component gas making up the source gas to a level corresponding to the end tidal concentration of the gas X, whereby the apparatus is adapted to administer a source gas having a first gas composition.
