The three mammalian neurokinins, substance P, neurokinin A and neurokinin B, as well as some agonists selective for their respective receptors, NK-P, NK-A and NK-B, were tested in a variety of pharmacological preparations in order to evaluate if the biological responses of the various tissues were mediated by single or multiple receptor types. Previous observations that the dog carotid artery, the rabbit pulmonary artery and the rat portal vein are selective preparations respectively for SP, NKA and NKB were confirmed in the present study by showing that only the respective selective agonists were active on these tissues. Multiple functional sites were demonstrated in intestinal tissues (guinea pig ileum, rat duodenum), which apparently contain the three neurokinin receptors. A large number of NK-P, together with some NK-A receptor sites were found in the guinea pig and rat urinary bladder. Similarly, the guinea pig trachea and the rabbit mesenteric vein contain NK-A and NK-P functional sites. Rat and rabbit vas deferens stimulated electrically respond as typical NK-A preparations, since they are almost insensitive to SP or NKB selective agonists. A mixture of NK-A and NK-B receptor sites has been shown to be present in the hamster urinary bladder: dog and human urinary bladder definitely contain NK-A receptors and the dog bladder also some NK-P functional sites.