The tachykinins (TKs) are a family of small peptides which share the common C-terminal sequence Phe-X-Gly-Leu-MetNH2. Three peptides of this family, substance P, neurokinin A and neurokinin B, have an established role as neurotransmitters in mammals. 2. Three receptors for TKs have been cloned: they are G-protein coupled receptors with seven putative transmembrane spanning segments and have been termed NK1 (substance P-preferring), NK2 (neurokinin A-preferring) and NK3 (neurokinin B-preferring). 3. Synthetic agonists are available to selectively stimulate only one receptor, while natural TKs can act as full agonist at each one of the three receptors, albeit at different concentrations. 4. A number of potent and selective antagonists, both peptide and nonpeptide in nature, have recently been developed. 5. The introduction of these ligands has revealed an unforeseen pharmacological heterogeneity of NK1, NK2 and NK3 receptors which appears largely, if not exclusively, linked to the existence of species homologues of the three receptors.