Ventilator apparatus provides a breathing circuit for supplying breathing gas to a patient. The breathing circuit includes a variable volume enclosure (3) provided within an enclosure chamber (2) and a gas supply path (21) into the enclosure chamber enabling the enclosure chamber to be pressurized by the supplied gas in order to vary the volume of the enclosure. An alternative path (9) for the gas supply is provided enabling the gas supplied, in predetermined circumstances, to enter directly into the breathing circuit via a valve (4). The gas supply path into the chamber is directed such that as a result of gas passing into the enclosure chamber the valve is biased more firmly to a closed condition.
