Neuropeptide Y (NPY), acting through various medial hypothalamic nuclei, is found to have potent effects on a variety of endocrine, physiological and behavioral systems that modulate energy balance. This peptide affects the release of various hormones, such as corticosterone, insulin, aldosterone and vasopressin, which modulate energy metabolism, as well as food intake. It also has direct impact on energy metabolism through an effect on substrate utilization and lipogenesis. Finally, NPY has a remarkably potent stimulatory effect on feeding behavior, which is characterized by a selective increase in carbohydrate ingestion that is strongest at the beginning of the active feeding cycle and is dependent upon circulating levels of corticosterone. This evidence has led to the proposal that NPY exerts anabolic effects to restore energy balance at specific times of energy depletion. Increased NPY activity may occur at the beginning of the active cycle or after a period of food deprivation. Further evidence, that chronic NPY stimulation produces profound hyperphagia and obesity and that endogenous NPY concentration is increased in genetically obese animals, strongly suggests that hypothalamic NPY may contribute to the development of eating disorders and obesity.