An anaesthetic and ventilatory system based on a partial rebreathing system (Mapleson D), for use in cat experiments, is described. A new “bag‐in‐box” excess valve construction is also presented. Factors influencing the Pco2of the Mapleson D system are discussed, as well as the practical application and the advantages of using this combination. Examples of the ability to reach different levels of Pco2ar