Short-chain fatty acids (SCFAs), including acetate, propionate and butyrate, are the most commonly found anions found in the monogastric mammalian large intestine, and are known to have a variety of physiological and pathophysiological effects on the gastrointestinal tract. We investigated the protein and mRNA expression levels of GPR41, a possible G protein coupled receptor for SCFA, using Western blot analysis and reverse transcriptase-polymerase chain reaction. We found that GPR41 protein and mRNA are expressed in human colonic mucosa. Immunohistochemistry for GPR41 showed that mucosal GPR41 protein is localized in cytoplasm of enterocytes and enteroendocrine cells. Moreover, GPR41-immunoreactive endocrine cells contained peptide YY but not serotonin or GPR43. The cellular population of GPR41 (0.01 ± 0.01 cells/crypt) was much smaller than that of GPR43 (0.33 ± 0.01 cells/crypt) in the human colon. However, the potency order of SCFA-induced phasic contraction of colonic smooth muscle that we previously reported is consistent with GPR41 (propionate ≧ butyrate > acetate) but not GPR43 (propionate = butyrate = acetate). Therefore, the present study suggests that GPR41 expressed in human colonic mucosa may function as a sensor for luminal SCFAs.Short-chain fatty acids (SCFA) are a major anion present in the large intestinal lumen of monogastric mammals including humans. SCFAs are produced during anaerobic bacterial fermentation of unabsorbed carbohydrates and dietary fibers. The concentration of SCFAs in human feces is reported to be approximately 100 mM, and are primarily comprised of acetate, propionate, and butyrate. Molar ratios of SCFAs in human fecal content are 50-60, 15-20, and 10-20 for acetate, propionate, and butyrate respectively (3, 22). Through absorption and metabolism of SCFAs, the host is able to salvage energy from food not digested in the upper intestine. Furthermore, luminal SCFAs have various physiological and pathophysiological effects in the gastrointestinal (GI) tract (4,10,17,19). In our previous studies, we demonstrated that SCFAs evoke two different effects on rat colonic smooth muscle in vitro (13)(14)(15). In the early phase, propionate and butyrate induced concentrationdependent phasic contractions in circular and longitudinal muscle, but acetate had no such effect (13-15). In the late phase, propionate induced a concentration-dependent increase in frequency of spontaneous contractions in circular (13) and longitudinal muscle (15), but butyrate did not affect the frequency of spontaneous longitudinal muscle contractions (15). On the other hand, acetate induced a