GPR40 was originally isolated from human genomic DNA by degenerate PCR. We isolated GPR40 cDNAs from various species, and precisely analyzed its mRNA expression in rat tissues, and found that GPR40 was highly expressed in beta cells in the islets of rat pancreas. When compared to the cell-surface receptors (i.e., choresistokinin receptor, glucagon-like peptide-1 receptor, and sulfonylurea receptor) that are known to predominantly express in the pancreatic beta cells, GPR40mRNA was comparable to these receptors in mRNA expression levels. In addition, all of pancreatic beta cell lines, which we examined, expressed GPR40mRNA at significant levels. Its highest expression was detected in a mouse beta cell line MIN6. To reveal the function of GPR40, we searched for the ligands of GPR40 by screening more than 1500 compounds. As a result we found that CHO cells expressing GPR40 specifically responded to free fatty acids (FFAs), that is, elevation of intracellular Ca(2+) was detected in these cells. Among FFAs tested, apparent stimulatory activities were detected in C12- to 16-length saturated FFAs (e.g., lauric acid, myristic acid, and palmitic acid) and in both C18- and C22-length unsaturated FFAs (e.g., oleic acid, elaidic acid, linoleic acid, a-linolenic acid, g-linolenic acid, arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid) at EC(50) of micro molar range. We found that FFAs induced Ca(2+) influx and activated MAP kinase in CHO cells expressing GPR40. As it is known that the increase of intracellular Ca(2+) promotes insulin secretion, we expected the stimulation of FFAs through GPR40 would promote insulin secretion from pancreatic beta cells. As we expected, FFAs induced glucose-stimulated insulin secretion (GSIS) in MIN6 cells. Our results indicate that GPR40 is a cell-surface receptor for FFAs and regulates insulin secretion from pancreatic beta cells. FFAs are known not only to provide an important energy source as nutrients for the body but also to act as signaling molecules in various cellular processes including insulin secretion. However, the molecular mechanism behind the relationship between insulin secretion and FFAs is little understood. We believe that the discovery of a cell-surface FFA receptor on pancreatic beta cells will provide a clue to resolve the relation between FFAs and insulin secretion, and thus eventually lead to the development of anti-diabetic drugs.