The role of vitamin E in the endocrine system, in particular the pituitary-gonadal axis, was studied in humans and male rats by examining the hormonal differences between vitamin E deficient and supplemented conditions. In vitamin E deficient rats, pituitary content and basal plasma level of FSH and LH were significantly lower than those of the control rats, but testicular content and basal plasma level of testosterone were not significantly changed. On the other hand, in vitamin E supplemented rats, FSH and LH content in pituitary tissue was significantly higher than that of the controls, but there was no significant rise in basal FSH and LH level in plasma. The testosterone level was significantly elevated in both testicular tissue and plasma. It was also demonstrated that basal plasma testosterone and F.T.I. were increased in normal male subjects following oral vitamin E administration and the responsiveness of plasma testosterone levels to HCG was significantly higher during vitamin E administration than before administration. These results suggest that vitamin E may play an important and potent role in hormone production in the pituitary-gonadal axis in humans and rats.