SUMMARY: In the hypophysectomized-ovariectomized-adrenalectomized hooded Norway rat, growth of the mammary ducts can be induced with oestrone + somatotrophin + corticoids; lobule-alveolar development ensues if in addition to the above combination progesterone + prolactin is also administered. On withdrawing these hormones the local injection of prolactin then causes secretion in the underlying mammary gland, but only when corticoids are administered systemically.