Leptin is a hormone of molecular weight of about 16 kDa consisted of 167 amino acids. It is produced mainly by white adipose tissue but its receptors are present also in other tissues and organs. Leptin is recognized as a sensing hormone factor regulating signal responsible for satiety acting as inhibitor of neuropeptide-Y release in hypothalamus generating reduced food intake and thereby a decrease of body weight. Leptin influences the reproductive system and is indispensable in sexual pubescence, its presence is important for proper function of female and male reproductive systems. Concentration of leptin in animal blood is correlated with fat deposition, back fat thickness and intramuscular fat suggesting possibility of its applying as a test of selection in pig, cattle and sheep meat quality evaluation. Leptin is synthesized in mammary gland and its concentration in colostrum and milk depends on the lactation stage. The results of experiments on piglets suggest that the presence of leptin in colostrum and milk influences the development and maturation of digestive tract in suckling piglets.