Growing rats during intermittent exposure to altitude stress of 5486.4 m, lost weight; the loss during a 2-week period was 50% compared to controls. When the stress was withdrawn the rats began to gain weight normally. The activities of proteolytic enzymes (pepsin and trypsin) were found to be significantly increased under the influence of stress. In thyroxine-injected guinea pigs (16 μg/kg) the activities are more pronounced.