Leptin is a newly discovered hormone; well known as metabolism modifier; secreted by adipose tissue. The role/s of leptin in mammary gland function still inscrutable and unclear. Current book sheds the light on the Hormonal profile of leptin around parturition in blood plasma and milk serum, and its relationship to blood plasma components and mammary gland function; in addition to the relation between plasma leptin and milk serum leptin concentrations in Friesian dairy cows under the Egyptian conditions. It is likely that leptin directs nutrients towards organs or tissues that are metabolically more active as well mammary gland, functioning as an autocrine or paracrine regulating signal. Leptin is an interesting hormone for scientists and pharmaceutical companies to manipulate; wishing to improve productive performance of animals. Furthermore, if leptin is involved in animal responses to disease or stress, further manipulation may become an important therapy in the treatment of animal disease. However, direct leptin treatment will not be feasible in the field of livestock production unless affordable, potent analogs or delivery systems are well developed.