Summary1. In most species oestrogens mainly stimulate mammary duct growth. In the guinea‐pig and goat they promote complete alveolar development. The mouse is the only animal in which treatment with progesterone has been found to promote mammary duct growth. In other species simultaneous treatment with oestrogens and progesterone is necessary for complete alveolar development. It is difficult to say whether or not oestrogens per se can promote alveolar development, because the adrenal cortex produces progesterone.2. Androgens cause duct and alveolar growth in some species.3. Some now believe that, under the influence of oestrogen, the anterior pituitary produces a “mammogenic” hormone which promotes mammary duct growth.4. The presence of the anterior pituitary is essential for the initiation and maintenance of lactation. The anterior lobe is believed to secrete a specific lactogenic hormone (prolactin), but other anterior lobe hormones probably participate in the control of normal lactation.5. Oestrogens may inhibit lactation during pregnancy, but it is probable that the pregnant uterus is also concerned in the inhibition.6. The adrenal‐cortical hormone is necessary for lactation, since adrenal‐ectomized animals do not lactate, and, further, prolactin will not initiate or maintain lactation in hypophysectomized animals unless corticosterone or adrenotrophic hormone is also given.7. The thyroid gland is essential for normal lactation: thyroxine stimulates nilk secretion in the cow.8. Posterior pituitary extracts cause emptying of the bovine udder but do not affect the formation of milk.9. Oestrogen treatment enriches milk in fatty and non‐fatty solids, and so does thyroxine treatment. Repeated injections of certain prolactin extracts increase the fat content of Cow's milk; certain other prolactin preparations increase the lactose content.10. No true secretory nerves innervate the mammary gland. Lactation is disturbed in the cat if the sympathetic chains are extirpated.11. The extraction of milk from the bovine udder depends on the reflex erection of udder tissues caused by stimulation of the teat, which also appears to cause the release of a lactogenic hormone or hormones from the hypophysis.12. The mammary gland synthesizes lactose from blood glucose and also, probably, lactic acid. It is also possible that blood amino acids are de‐aminated by the mammary gland and used for lactose synthesis.13. The precursor of milk fat is the neutral fat of the blood. The mammary gland probably oxidizes long‐chained fatty acids to the short‐chained acids of milk fat.14. It is improbable that milk protein arises from blood amino acids. Plasma proteins are probably utilized for the synthesis of caseinogen, the phosphorus coming from the blood inorganic phosphate.15. Various enzymes have been detected in active mammary tissue, which is a particularly rich source of the alkaline phosphomonoesterase. Changes in the secretory function of the mammary gland are usually accompanied by striking changes in the phosphomonoesterase content of milk.