The mechanism of estrogen action on CBG level in blood serum was studied in the experiments on white rats. It was shown that estradiol injection at the dose 100 micrograms for 3 weeks induced the increasing of CBG serum level in intact males, but it was not changed in the gonadectomized males and females. The presence of the positive effect of exogenic estrogens only in the intact males was connected with the estrogen depression of testis function. It suggests the absence of direct positive estrogens action on the level of CBG in the rats. The estrogen effect is not mediated by adrenal corticosteroids, and has independent negative action on CBG level in the rat. Thus, estrogen does not participate in the endocrine regulation of CBG content in the mature rats.