The mammary glands of virgin [female] or dry goats were greatly increased in size over the control condition by treatment with diethylstilbestrol and diethylstilbestrol dipropionate pellets. In 5 cases histologic examination revealed fairly normal, although not complete, lobule-alveolar development after long-continued treatment as compared to the complex duct system found in control virgin goats. In 3 goats treated with diethylstilbestrol the histologic structure of the mammary glands was not that of normal alveolar lobules but consisted of solid masses of cells. In the case of a mature multiparous goat which gave as much as a liter of milk a day from one mammary gland, there was comparatively little epithelial development. Small isolated clusters of alveoli or ducts, and hypertrophied main ducts filled with secretion were present. [male] goats, in spite of heavy dosage and prolonged treatment, failed to respond to diethylstilbestrol dipropionate pellets placed subcut. An aged sterile cow had fairly well-developed lobule-alveolar systems in a normal as well as in 3 abnormal quarters of the udder after prolonged diethylstilbestrol treatment and lactation.