Young adult male and female Sprague-Dawley rats were exposed to a simulated altitude of 33,500 ft. both with and without restraint. The exposure was begun immediately upon the restraint of the experimental animals. The restrained animals died significantly sooner than did the nonrestrained controls. The possible relation of colonic temperature, oxygen consumption and emotional stress to the decreased altitude tolerance is discussed. It is suggested that the data serve as a warning for caution in the use of restraint for convenience in altitude tolerance experiments. Submitted on August 6, 1958