In rats fed a diet with 0.25 mg Se/kg the testis selenium content rose during maturation. The value in 4-mo-old animals (7.0 mg Se/kg dry mass; 0.9 mg Se/kg wet mass) was six times higher than that in 20-d-old weanling rats. By comparison, the selenium content in the main selenium pools, muscle and liver, remained unchanged and rose by half, respectively. Due to the increased selenium requirement of the testis during its pubertal maturation the amount of selenium taken up by the male gonads was 50% of the amount deposited in muscle and liver, whereas before and after that period it was about 10%. Feeding animals a low vitamin E diet had no effect on the rise in testis selenium. Glutathione peroxidase activity was twice as high in 4-mo-old animals as in weanling rats. Because only a small percentage of the element in the male gonads was bound to the enzyme, the rise in testis selenium must have been due to other selenium compounds. The selenium content of the spermatozoa was about 21 mg Se/kg dry mass, which by far exceeded the level of this element in other compartments of the rat. The increase in testis selenium content coincided with the beginning of spermatogenesis, and it may therefore ensure the supply of adequate amounts of the element for the spermatozoa.