Tests performed on spayed, oestradiol-primed rats with ligated uteri and normal pituitary function have shown that treatment with sexual steroids modifies uterine secretion. Progesterone elicits a decrease in the quantity and an increase in the viscosity of the uterine secretion, an effect also produced by testosterone at very high dose levels. A decrease in the pH of the secretion was also observed. Supplementary oestradiol treatment was devoid of any activity.This paper describes the results obtained when the same experiment was carried out in normal castrate and castrate-hypophysectomized rats. The effects of varying subcutaneous doses of progesterone, testosterone, and oestradiol were compared. In both groups, a number of rats primed with oestradiol only served as controls.Progesterone induced a dose-dependent reduction in the weight of the uterus and in the quantity and pH of the uterine secretion, as well as a marked increase in its viscosity, in both intact and hypophysectomized rats.Testosterone increased the weight of the uterus and, when given in large doses, reduced the pH of both the lumen and secretion. The quantity of the uterine secretion diminished in response to high doses and its viscosity increased markedly only in rats bearing a pituitary.Oestradiol did not elicit significant changes in either group; a slight reduction in the pH of the uterine secretion was observed in response to high doses.It may be concluded that the progestative modifications induced by progesterone in the uterus of spayed, oestradiol-primed rats, including particularly changes in uterine secretion, are the effects of a peripheral mechanism not involving the pituitary. Testosterone appears to be an exception as far as the quantity and viscosity of uterine secretion are concerned, since modifications in these parameters are only observed in the presence of a functional pituitary body.