Abstract: A constant volume rebreathing system when utilized in conjunction with an external air source, such as man. The rebreathing system forms a completely closed system between man and the environment. There is a stainless steel bellows within the system which in conjunction with ancillary equipment controls the differential pressure between man's lung and the bellows. A pressure transducer senses any pressure change and in accordance therewith alters the volume of the bellows, thereby assisting man's use of the rebreathing system. In addition analyzers are located within the system to monitor the various gases present.