The U.S. Army Research Institute of Environmental Medicine tested two samples each of three different types of rescue breathing apparatus (rba) to determine how long both could be worn by operators in subfreezing temperatures. Phase I tests were performed after overnight cold soaking at the test temperature; Phase II tests were performed after overnight storage at +20 deg. C. Successful performance was determined as proper operation for at least one-half the certified duration of the oxygen supply without exhaustion of the oxygen supply, increase of inspired co2 above physiologically acceptable levels, or increase in breathing resistance above tolerable levels. In Phase I, the scott "rescue pak r" operated successfully at -20 deg. C, while the drager 174-a and the McCaa units operated successfully at -10 deg. C. In Phase II, the scott unit operated successfully at -25 deg. C, the drager at -20 deg. C, and the McCaa at -15 deg. C. None of the rba types appeared well designed for use in subfreezing temperatures.