The report describes evaluation of two prototype phase dilution rebreathing masks as compared to an open port rebreathing mask design. Human subjects wearing the prototype masks and engaged in three minute periods of rest and exercise were exposed to altitudes of 14,000, 18,000, 25,000, 30,000 and 34,000 feet in an altitude chamber. Each subject was instrumented to obtain a variety of physiological parameters including end expiratory gas composition, blood oxygen saturation (ear oximetry), ECG, and respiratory rate (impedance pneumograph). Oxygen flow and system pressure as provided by an automatic constant-flow regulator were also determined. The automatic constant-flow regulator when used in conjunction with either of the prototype phase dilution rebreather test masks and proper orifices maintained the subjects' indicated blood oxygen saturations equal to or in excess of their ground level (1,273 feet) baselines. With the same oxygen flow rates the prototype porous and dilution valve masks provided higher tracheal oxygen partial pressures than the open port masks at all altitudes tested during rest and exercise. The prototype phase dilution rebreather masks appear to be more efficient since at the same flow rates a greater amount of oxygen was derived from the supply and less oxygen lost to the ambient environment as compared to the open port type mask. (Author).