We have reported recently the development of substance P analogs which selectively activate different tachykinin receptor types. These compounds, namely septide [( pGlu6, Pro9]substance P6-11 hexapeptide), which preferentially acts on the NK-1 (SP-P) receptor, and senktide (succinyl-[Asp6,Me-Phe8] substance P (6-11) hexapeptide), which selectively acts on the NK-3 (or SP-N) receptor, were now used to identify the tachykinin receptors present in several smooth muscle systems. Septide as well as senktide were found to be devoid of activity in the electrically stimulated rat vas deferens, showing that, in this preparation, which is the prototype NK-2 (or SP-E) system, tachykinin-induced contraction is not mediated via NK-1 or NK-3 receptors. In contrast, senktide did evoke contraction of the isolated rat duodenum, demonstrating that this tissue, which had been classified previously as a NK-2 preparation, also contains NK-3 receptors. After desensitization of NK-3 receptors by treatment with a desensitizing concentration of senktide, the rank order of tachykinin potencies in contracting the isolated rat duodenum corresponded to that of a typical NK-2 system. In the guinea pig ileum in which the tachykinin-evoked contraction is mediated by both NK-1 and NK-3 receptors, desensitization of NK-3 receptors unmasked a typical NK-1 response. Inasmuch as selective desensitization of both NK-1 and NK-3 sites abolished the tachykinin-induced response of the guinea pig ileum, it seems that this preparation does not contain NK-2 receptors. Our results show that selective agonists can be used to perform independent assays of several tachykinin receptors present within the same tissue preparation.(ABSTRACT TRUNCATED AT 250 WORDS)