The amount of pentane in the breath of rats in a vitamin E (VE) -deficient group was found to be significantly higher than that in a VE-supplemented group. The activities of antioxidative protective enzymes from the lungs of VE-deficient animals were found to be lower than those of VE-supplemented animals. By addition of selenium to the diet, the amount of pentane in the breath showed a decreased tendency, as compared with that in selenium-unsupplemented groups. From the results of these experiments, the amount of pentane exhalation, TBA value and the amount of VE in tissues or blood plasma were found to be significantly related to each other. In humans, a significant relationship between plasma VE value and pentane exhalation in breath was confirmed. Since administration of antibiotics to rats did not affect the amount of pentane in the breath, it is assumed that this exhaled pentane is not produced by the action of enterobacteria.