The endogenous ligand for growth-hormone secretagogue receptor (GHS-R) was purified from the stomach and we named it "ghrelin", after a word root ("ghre") in Proto-Indo-European languages meaning "grow", since ghrelin has potent growth hormone (GH) releasing activity. In addition, ghrelin stimulates appetite by acting on the hypothalamic arcuate nucleus, a region known to control food intake. Ghrelin is orexigenic; it is secreted from the stomach and circulates in the blood stream under fasting conditions, indicating that it transmits a hunger signal from the periphery to the central nervous system. Taking into account all these activities, ghrelin plays important roles for maintaining growth hormone release and energy homeostasis in vertebrates. The diverse functions of ghrelin raise the possibility of its clinical application for GH deficiency, eating disorder, gastrointestinal disease, cardiovascular disease, osteoporosis and aging, etc.