A loop system exists between hypothalamic neuropeptide Y (NPY) and peripheral adipose tissue leptin to maintain normal body homeostasis. When hypothalamic NPY levels are increased by fasting or by intracerebroventricular (i.c.v.) infusion, food intake and body weight increase. NPY has genuine hormono-metabolic effects. It increases insulin and corticosterone secretion relative to controls. These hormonal changes, acting singly or combined, favor adipose tissue lipogenic activity, while producing muscle insulin resistance. They also promote leptin release from adipose tissue. When infused i.c.v. to normal rats to mimic its central effects, leptin decreases NPY levels, thus food intake and body weight. Leptin i.c.v. has also genuine hormono-metabolic effects. It decreases insulinemia and adipose tissue storage ability, enhancing glucose disposal. Leptin increases the expression of uncoupling proteins (UCP-1, -2, -3) and thus energy dissipation. Leptin-induced changes favor oxidation at the expense of storage. Circadian fluctuations of NPY and leptin levels maintain normal body homeostasis. In animal obesity, defective hypothalamic leptin receptor activation prevent leptin from acting, with resulting obesity, insulin and leptin resistance.