There are two properties of vitamin E which are leading to its use for better physical performance. On one side it promotes an economical energy metabolism, on the other side it acts as a stabilizing antioxidant in membranes. One group of 6 high altitude mountain climbers was substituted with 2 x 200 mg dl-alpha-tocopheryl acetate and a second of 6 were given placebos during a 10 weeks expedition. The influence on the synthesis of lactic acid and on the exhalation of pentane was investigated. After 2 weeks the anaerobic threshold (AT) was higher in both groups. In the course of the experiment the AT of the treatment group increased further whereas the AT of the control group decreased significantly (p less than 0.01). Pentane exhalation, considered to be a measure for lipid peroxidation, showed no significant change after a 4 weeks substitution with vitamin E in the treatment group, but in the control group the exhaled pentane was more than 100% higher. The difference is significant, too (p less than 0.01). The results of this study show that vitamin E has a beneficial effect on physical performance and on cell protection, at least at high altitude.