The taste system, made up of taste receptor cells clustered in taste buds at the surface of the tongue and the soft palate, plays a key role in the decision to ingest or reject food and thereby is essential in protecting organisms against harmful toxins and in selecting the most appropriate nutrients. To determine if a similar chemosensory system exists in the gastrointestinal tract, we used immunohistochemistry and real-time polymerase chain reaction (PCR) to investigate which taste-signaling molecules are expressed in the intestinal mucosa. The PCR data showed that T1r1, T1r2, T1r3, alpha-gustducin, phospholipase Cbeta2 (PLCbeta2), and Trpm5 are expressed in the stomach, small intestine, and colon of mice and humans, with the exception of T1r2, which was not detected in the mouse and human stomach or in the mouse colon. Using transgenic mice expressing enhanced green fluorescent protein under the control of the Trpm5 promoter, we found colocalization of Trpm5 and alpha-gustducin in tufted cells at the surface epithelium of the colon, but these cells did not express T1r3 or PLCbeta2. In the duodenal glands, 43%, 33%, and 38% of Trpm5-expressing cells also express PLCbeta2, T1r3, or alpha-gustducin, respectively. The duodenal gland cells that coexpress PLCbeta2 and Trpm5 morphologically resemble enteroendocrine cells. We found a large degree of colocalization of Trpm5, alpha-gustducin, T1r1, and T1r3 in tufted cells of the duodenal villi, but these cells rarely expressed PLCbeta2. The data suggest that these duodenal cells are possibly involved in sensing amino acids.