The principal lactogenic hormone, prolactin, secreted by the anterior pituitary is critical to the establishment of lactation, milk macronutrient content and milk production. The concentration of circulating prolactin increases during pregnancy so that by the end of gestation, levels are 10 to 20 times over normal amounts. However, prolactin is prevented from exerting its effect on milk secretion by elevated levels of progesterone. Following clearance of progesterone and estrogen at parturition, copious milk secretion begins. The minimal hormonal requirements for normal lactation to occur are prolactin, insulin and hydrocortisone. Prolactin stabilizes and promotes transcription of casein mRNA; may stimulate synthesis of alpha-lactalbumin, the regulatory protein of the lactose synthetase enzyme system; and increases lipoprotein lipase activity in the mammary gland. Prolactin levels decrease as lactation is established but nursing stimulates prolactin release from the pituitary which promotes continued milk production. Prolactin is secreted into milk at levels representative of the average circulating concentration. The physiological significance of milk prolactin to the infant is uncertain. Prolactin exists in three heterogenic forms which possess varying biological activity. The monomer with a molecular weight of 23 kDa is found in greatest quantity and is the principal biologically active form. The pattern of heterogeneity changes during pregnancy to favor even more monomer in proportion to the dimer. However, during lactation, the proportion of the monomer in circulation decreases in response to selective uptake of the monomer by the mammary gland. Over 90 percent of the prolactin in milk is present as the monomer. Prolactin may exert some of its biological effect by a shift in the ratio of active to less active forms of the molecule.