Molecular Genetics, Institute of Life Science, Kurume University, Kurume, Fukuoka; Department of Biochemistry, National Cardiovascular Center Research Institute, Suita, Osaka; and Translational Research Center, Kyoto University Hospital, Kyoto, Japan Small synthetic molecules called growth hormone secretagogues (GHSs) stimulate the release of growth hormone (GH) from the pituitary. They act through the GHS-R, a G protein-coupled receptor whose ligand has only been discovered recently. Using a reverse pharmacology paradigm with a stable cell line expressing GHS-R, we purified an endogenous ligand for GHS-R from rat stomach and named it "ghrelin," after a word root ("ghre") in Proto-Indo-European languages meaning "grow." Ghrelin is a peptide hormone in which the third amino acid, usually a serine but in some species a threonine, is modified by a fatty acid; this modification is essential for ghrelin's activity. The discovery of ghrelin indicates that the release of GH from the pituitary might be regulated not only by hypothalamic GH-releasing hormone, but also by ghrelin derived from the stomach. 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 bloodstream 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 GH release and energy homeostasis in vertebrates. Address for reprint requests and other correspondence: Address for reprint requests and other correspondence: M. Kojima, Molecular Genetics, Institute of Life Science, Kurume University, Hyakunenkouen 1–1, Kurume, Fukuoka 839–0864, Japan (E-mail: mkojima{at}lsi.kurume-u.ac.jp )