Using rat isolated superior cervical ganglion we have further characterised tachykinin NK 1 receptors and investigated the possible existence of tachykinin NK 1 receptor subtypes. At 37 °C, tachykinin NK 1 receptor antagonists GR82334 ([ d-Pro 9[spiro-γ-lactam]Leu 10,Trp 11]physalaemin-(1–11)), CP-99,994 ((+)-(2 S,3 S)-3-(2-methoxybenzylamino)-2-phenylpiperidine) and (±)-RP67580 (7,7-diphenyl-2[1-imino-2(2-methoxy-phenyl)-ethyl]perhydroisoindol-4-one (3 aR,7 aR)) antagonised more potently depolarisation responses evoked by GR73632 (δAva[ l-Pro 9, N-MeLeu 10]SP-(7–11)), septide ([pGlu 6,Pro 9]SP-(6–11)) and neurokinin A than those evoked by substance P, substance P O-methyl ester and [Sar 9,Met(O 2) 11]substance P. GR73632 and substance P O-methyl ester evoked depolarisation responses of similar magnitude, unaffected by addition of tetrodotoxin, but which cross-desensitised. At 22 °C, the ability of GR82334 and (±)-RP67580 to inhibit substance P O-methyl ester-evoked but not GR73632-evoked responses was enhanced greatly. These results suggest a single population of tachykinin NK 1 receptors in this preparation. The agonist and temperature dependency of tachykinin NK 1 receptor antagonist potency in rat isolated superior cervical ganglion may reflect different conformational changes in the tachykinin NK 1 receptor induced by partial or full sequence substance P analogues.