Abstract: Breathing apparatus is disclosed in which a selected portion of each exhalation is returned to the inlet side for rebreathing. The percentage of each exhalation rebreathed varies from exhalation to exhalation and is determined in accordance with the detected magnitude of the preceding inhalation. Breathing mixture is withdrawn by the user from a reservoir which is in communication with a cylinder having a spring loaded piston therein. The pressure in the reservoir, and hence in said cylinder, at the end of each inhalation determines the position of the piston. A chamber receives the exhaled breath and expands to a degree dependant on the position of the piston. Once the chamber has expanded to the maximum extent permitted for that exhalation, the remainder of the exhalation is discharged from the apparatus.