Neurokinins are a group of peptide compounds occurring in the nervous system of mammals. They are characterized by a common sequence of four amino acids at the C-terminal of a peptide chain. The neurokinins include substance P (SP), neurokinin A (NKA) and neurokinin B (NKB). The neurokinins bind to their specific receptors NK1, NK2 and NK3. An amino acid sequence of these receptors is over 65 per cent homologous. SP is a ligand of the NK1 receptor, NKA - of the NK2 receptor, and NKB - of the NK3 receptor. Each of them is simultaneously an agonist of neurokinin receptors of every type.