SUMMARY: Mammogenesis has been studied in hypophysectomized goats (nine completely and four incompletely hypophysectomized). In the absence of the pituitary, hexoestrol alone or in combination with progesterone failed to stimulate the growth of the mammary gland or to prevent regression of the already developed mammary gland. In the ovariectomized-hypophysectomized goat considerable mammary lobulo-alveolar growth can be promoted by administering hexoestrol and progesterone in combination with prolactin (ovine), growth hormone (bovine) and adrenocorticotrophin.