Mammary glands, besides being among the tissues in the body with the highest lipid content after adipose tissue (1), are one of the most active metabolic tissues in the body during pregnancy and lactation. Milk lipid is an important source of both calories and essential fatty acids for the newborn. During lactation, women secrete 800 ml of milk per day containing 4% fat, mostly corresponding to triacylglycerols, of which the mammary gland synthesizes ف 32 g daily (2). The lactating mouse mammary gland secretes 5 ml of milk per day containing ف 30% fat (3). To develop this transitory capacity for handling such a large amount of lipids, the morphology of the mammary tissue changes during pregnancy and lactation. In nonpregnant mammary glands, the predominant cells are adipocytes with epithelial structures interdispersed among them. During pregnancy, in mammary glands there is an extensive proliferation of alveolar structures into the adipocytes, accompanied by differentiation of epithelial cells (4), which show cytoplasmic lipid droplets (5) surrounded by the protein adipophilin (6). During lactation, epithelial cells are the predominant cell type, and only small channels of lipid-filled adipocytes and lipid-depleted adipocytes may be distinguished.During pregnancy, major changes in maternal lipid metabolism occur. Fat depots accumulate during the early stages of pregnancy and decrease during the late phases (7). Net catabolic changes taking place in adipose tissue during late pregnancy are manifested by an enhanced hormone-sensitive lipase (HSL) activity and decreased LPL activity (8), which result in an increase in maternal plasma lipids both in humans (9, 10) and in rats (11,12). The increments of triglyceride (TG)-rich lipoproteins (chylomicrons and VLDLs) are among the most pronounced changes in plasma lipids during late pregnancy (10,13,14). Reduced adipose tissue LPL activity during late gestation allows blood TG to be diverted from storage in adipose tissue to other tissues, such as mammary glands, where there is an induction in LPL expression and activity, allowing the subsequent hydrolysis and uptake of circulating TG in preparation for lactation (15,16).