Inhibition of Mammary Growth by High Doses of Estrogen **

The administration of large doses of estrogen to young male mice was followed by less extensive mammary gland growth than when small amounts of estrogens were given". The areas occupied by the mammary glands of the mice given large doses were often but little larger than the glands of untreated controls, but unlike the controls their mammary glands, although very small, had a few alveoli on the short, distended mammary ducts. Mammary glands of restricted size were also observed in monkeys,2" rabbits,' dogs,' and rats' given large doses of estrogens. The area of proliferation of mammary glands of male mice was inversely proportional to the amount of estrogens given beyond a certain optimal amount.' The restricting effect of large doses of estrogen upon the area of nonneoplastic mammary growth might be compared to the effects of large doses of estrogen on neoplastic growth, especially in post-menopausal women.8 In neither instance has the mechanism for the inhibition or restriction of growth been explained adequately. Whether the large amounts of estrogen inhibit the growth of mammary parenchyma by direct action or action upon the stroma, or whether the effect is upon the pituitary gland or adrenal glands and hence upon the mammary glands indirectly, is not known. Some experiments to determine how large doses of estrogens inhibited mammary growth were undertaken.