Tachykinins are peptides sharing a common C-terminal amino acid sequence: Phe-X-Gly-Leu-Met-NH2. This neuropeptide family is composed of substance P, neurokinin A, and neurokinin B, which are widely distributed in mammalian central and peripheral nervous systems. These three molecules serve as both neurotransmitters and neuromodulators. Their actions are mediated by binding with three distinct receptors, namely NK1, NK2, and NK3. In particular, NK2 is expressed in gastrointestinal tract. Activation of NK2 is chiefly responsible for the regulation of intestinal motor functions (both excitatory and inhibitory), secretions, inflammation, and visceral sensitivity. Antagonists of NK2 may be useful in the trea™ent of irritable bowel syndrome.