An apparatus for use in oxygen therapy can be constructed so as to include a gas holder appearing more or less as a pendant which includes a variable volume internal chamber and so as to use a line for continuously delivering oxygen to the holder and a conduit for conveying gas between a person using the apparatus and the holder. During use on exhalation the initially exhaled gas will flow through cannula carried by the conduit and the conduit into the holder so as to expand the variable volume chamber. When it is filled the remaining expired gas will be vented to the ambient. As the holder is filled the expired gas within it will be mixed with a limited amount of oxygen and thereafter during exhalation the oxygen supply will displace the expired gas in the conduit so that it flows outwardly from the cannula to be picked up by the expired air being vented to the ambient. On inhalation the oxygen and any non-displaced gas in the conduit will be inhaled preferentially to the ambient air and will be followed into the respiratory track by the gas from within the variable volume chamber and the oxygen concurrently supplied to the holder. During the remainder of inhalation ambient air will be inhaled.
