1. Following the recent proposal that the selective agonist septide, ([pGlu6,Pro9]SP(6-11)), acts on a novel tachykinin receptor distinct from the 'classical' NK1 receptor, the aim of the study was to investigate the possible heterogeneity of tachykinin NK1 receptors in the rat urinary bladder. 2. The synthetic tachykinin receptor agonists, septide (pD2 7.87) and [Sar9]substance P (SP) sulphone (pD2 7.64) produced concentration-dependent contractions of the rat isolated urinary bladder. 3. The NK1 receptor antagonists GR82,334, (+/-)-CP96,345, and RP67,580 competitively antagonized (slopes of Schild plot not significantly different from unity) the response to septide with the rank order of potency (pKB values in parentheses): RP 67,580 (7.57) > GR 82,334 (7.01) > (+/-)-CP 96,345 (6.80). The same antagonists were significantly less potent when tested against [Sar9]SP sulphone, while maintaining the same rank order of potency: RP 67,580 (7.00) > GR 82,334 (5.93) > (+/-)-CP 96,345 (< 6). The antagonists did not affect the concentration-response curve to bombesin. 4. To exclude the involvement of the NK2 receptor, a second series of experiments was performed in the presence of the potent nonpeptide NK2 receptor antagonist, SR 48,968. SR 48,968 (1 microM) produced a rightward shift of the concentration-response curve to the NK2 receptor selective agonist, [beta Ala8]neurokinin A (NKA) (4-10). SR 48,968 did not significantly modify the response to SP, NKA, neurokinin B (NKB), neuropeptide K (NPK), neuropeptide gamma (NP gamma), SP(4-11), SP(6-11), septide or [Sar9]SP sulphone. 5. In the absence or presence of SR 48,968, RP 67,580 antagonized in a competitive manner the response to septide, [Sar9]SP sulphone, SP(4-11) and SP(6-11): pKB values obtained in the absence and presence of SR 48,968 were not significantly different for any of these four agonists.6. RP 67,580 antagonized the response to SP and NKA both in the absence and presence of SR 48,968.In both cases, the slopes of the Schild plots were significantly different from unity. Mean dose-ratios produced by RP 67,580 in the presence of SR 48,968 were larger than those measured without NK2receptor blockade for both SP and NKA.7. RP 67,580 (3 MicroM) did not antagonize the response to NKB in the absence of SR 48,968. In the presence of SR 48,968, RP 67,580 acted as a competitive antagonist of NKB-induced contractions with apKB value (7.63) not significantly different from that measured towards septide. In the present of SR48,968, RP 67,580, GR 82,334 and (+/-)-CP 96,345 antagonized the response to NKB with a rank order of potency identical to that measured towards septide or [Sar9]SP sulphone.8. In the absence of SR 48,968, RP 67,580 (3 MicroM) produced a small shift of the concentration-response curve to neuropeptide K and was ineffective toward neuropeptide T. In the presence of SR 48,968 a clear shift of the curve to both agonists was observed.9. These findings are compatible with the idea that a septide-sensitive tachykinin receptor may exist in the rat urinary bladder. The septide-sensitive receptor is recognized by NK1 receptor antagonists with higher affinity than the 'classical' NK1 receptor recognized by [Sar9]SP sulphone. Our data suggest that NKB, after NK2 receptor blockade, is a more suitable ligand than SP for activation of the 'septidesensitive'receptor. While the final proof for the existence of possible NK1 receptor subtypes must await confirmation at the molecular level, the present findings provide strong pharmacological evidence that either NK, receptor subtypes or a novel type of tachykinin receptor exist in the rat urinary bladder.