The mammary gland growth--appearance of a lobulo-alveolar structure--, the secretion of colostrum and lactogenesis occur when precise endocrine equilibrium take place during gestation and lactation in the cow and the sow. The formation of alveoli requires hormonal sequences including first, ovarian and foetoplacental hormones--estrogens and progesterone--and, then, antepituitary--prolactin--and adrenal--corticoids--hormones. These sequences appear during pregnancy and lead to a near complete development of the mammary gland at parturition in the cow and the sow. Administrations of ovarian steroids which produce the same variations of levels of these hormones in plasma as during the pregnancy allow the lobulo-alveolar structure to develop in the non pregnant dried cow. The synthesis of specific products of milk--casseins and lactose--remains low throughout pregnancy and then increase sharply after calving or farrowing. Around parturition, the secretion of colostrum takes place when plasma levels of progesterone drop very fast and those of estrogens increase and are at the highest level observed during gestation. A few hours later, the plasma levels of prolactin and corticoids increase significantly. The colostrum secretion, the appearance of high affinity IgG1 receptors and the specific uptake of IgG1 in maternal serum coincide with a complicated hormonal environment in which a lower progesteronemia and a higher prolactinemia seem to play a major role. Estrogens--especially 17 beta-estradiol--are required for the apperance of new epithelial mammary cells which acquire specific binding sites for IgG1 later. After injections of 17 beta-estradiol and progesterone to non pregnant, dried cows, the IgG1 secretion takes place when the plasma levels of these steroids decrease. On the other hand, the secretion of colostrum is the same as in a normal parturition when calving is induced by dexamethasone or dexamethasone + estradiol benzoate injections. After parturition, there is a lower uptake of proteins from the serum when prolactin and corticoids induce the onset of copious mileticulum, of the Golgi apparatus, of mitochondria and of the appearance of a polarized structure which depress the possibilities of migration of proteins from the serum through the cell.