The synthesis of prothrombin in hepatic microsomes is augmented in intact estrogen-treated rats and in hypophysectomized rats treated with purified prolactin. We investigated the influence of these gonadal and pituitary hormones on the titer of Hageman factor (factor XII), reportedly elevated in women using oral contraceptives. Rats were ovariectomized to minimize the influence of endogenous estrogen and progesterone on the Hageman factor titer. The administration of progesterone did not alter the plasma concentration of Hageman factor. In contrast, the infusion of 17 beta-estradiol induced a marked elevation of the plasma Hageman factor titer, as measured functionally and immunologically. The titer of Hageman factor was directly related to both plasma estradiol and prolactin concentrations, indicating that prolactin may play a role in the regulation of plasma Hageman factor titers. In agreement with this, hypophysectomy induced a marked decrease in the Hageman factor level. In hypophysectomized ovariectomized animals, the administration of estradiol restored the Hageman factor titer to normal levels, whereas the infusion of prolactin induced a dramatic rise in the Hageman factor titer to the degree observed in nonhypophysectomized estrogen-treated rats. No further increase in the Hageman factor titer was observed in rats treated with both estradiol and prolactin. These data indicate that estrogens increase the plasma Hageman factor titer both directly and through its release of prolactin and that prolactin may also increase the titer of Hageman factor through estrogen-independent mechanisms.