Tachykinin receptors are activated by the endogenous peptides substance P (SP), neurokinin A (NKA), neurokinin B (NKB). There are three tachykinin receptors; NK1, NK2, and NK3, and each has a preferred ligand; SP, NKA, and NKB, respectively. The term tachykinin was originally used to identify SP because it produced spasmodic effects in various intestinal smooth muscle cells. Tachy is Greek for rapid and Kinin is Greek for set in motion. Together these terms described the rapid contraction of smooth muscle treated with SP. NK1 and NK3 are widely distributed in the central nervous system, while NK2 is localized in the smooth muscle of the gastrointestinal, respiratory, and urinary tracts and some discrete regions of the CNS such as the prefrontal cortex and the hippocampus. Antagonists such as aprepitant and fosaprepitant were approved for the prevention of nausea and vomiting associated with cancer chemotherapy.