The endogenous ligand for the growth-hormone (GH) secretagogue receptor was purified from stomach and named ghrelin. It has potent GH-releasing activity and stimulates appetite by acting on the hypothalamic arcuate nucleus, a region known to control food intake. Ghrelin thus plays important roles in maintaining GH release and energy homeostasis in vertebrates. Ghrelin, moreover, stimulates gastric motility and acid secretion, shows positive cardiovascular effects, and has direct actions on bone formation. The diverse functions of ghrelin raise the possibility of its clinical application for GH deficiency, eating disorders, gastrointestinal disease, cardiovascular disease, osteoporosis and aging.