Control (physiological saline treated) and ascorbic acid (AA) treated (1 mg . g'1 b.w. one hour before exposure) 18-day-old rats were exposed for 1 hour to high altitude in a hypobaric chamber and the mean lethal altitudes were calculated. AA displayed a protective effect, so that in two identical experiments the mean lethal altitude was 10 900 and 10 150 m in controls, while it was 11 500 and 11 450 m in AA treated animals.