1. We studied the effect of the natural tachykinins and of synthetic agonists selective for the tachykinin NK1, NK2 and NK3 receptors, on the motility of guinea-pig isolated common bile duct longitudinally-oriented smooth muscle. 2. All the tachykinins tested (both natural and synthetic) produced a concentration-dependent contractile response of the guinea-pig isolated common bile duct: these effects underwent a marked tachyphylaxis, especially the responses elicited by NK1 and NK3 receptor-selective agonists. 3. Among the natural tachykinins neurokinin B (EC50 = 3.2 nM; 95% c.l. = 2.0-5.1; n = 4) was the most potent, being about 40 and 25 fold more potent than substance P (EC50 = 121.6 nM; 95% c.l. = 94-157; P < 0.01; n = 4) and neurokinin A (EC50 = 83.4 nM; 95% c.l. = 62-112; P < 0.01; n = 4), respectively. Among the synthetic analogues the NK3 receptor-selective agonist senktide (EC50 = 1.1 nM; 95% c.l. = 0.7-1.8; n = 8) was the most potent, being about 120, 110 and 20 fold more potent than [Sar9]substance P sulfone (NK1 receptor-selective) (EC50 = 130.4 nM; 95% c.l. = 99-172; P < 0.01; n = 8), [beta Ala8]NKA (4-10) (NK2 receptor-selective) (EC50 = 120.1 nM; 95% c.l. = 95-151; P < 0.01; n = 8) and septide (NK1 receptor-selective) (EC50 = 22.6 nM; 95% c.l. = 18-28; P < 0.01; n = 8), respectively. All tachykinins (natural or synthetic receptor agonists) produced a similar Emax, averaging about 50% of that produced by KCl (80 mM). 4. Atropine (1 microM) did not affect the responses to either NK1 or NK2 receptor-selective agonists, whereas it reduced the Emax of senktide by about 50%, without affecting its potency (EC50). Tetrodotoxin (1 microM) totally blocked senktide-induced contractions, as did the combined pretreatment with atropine plus the tachykinin NK1 and NK2 receptor-selective antagonists GR 82334 and MEN 11420 (1 microM each), respectively. 5. GR 82334 (1 microM) blocked with apparent competitive kinetics septide- (apparent pKB = 7.46 +/- 0.10; n = 5) and [Sar9]substance P sulfone- (apparent pKB = 6.80 +/- 0.04; n = 4) induced contractions. MEN 11420 (30-300 nM), a novel potent NK2 receptor antagonist, potently antagonized [beta Ala8]NKA (4-10), with competitive kinetics (pKB = 8.25 +/- 0.08; n = 12: Schild plot slope = -0.90; 95% c.l. = -1.4; -0.35). The NK3 receptor-selective antagonist SR 142801 (30 nM) produced insurmountable antagonism of the senktide-induced contractions (Emax inhibited by 64%). None of the above antagonists, tested at the highest concentrations employed against tachykinins, affected the concentration-response curve to methacholine (0.1-300 microM). 6. We conclude that tachykinins produce contraction of the guinea-pig isolated common bile duct by stimulating NK1, NK2 and NK3 receptors. The responses obtained by activating NK1 and NK2 receptors are atropine-resistant. The contraction obtained by stimulating NK3 receptors is totally neurogenic, being mediated by the release of endogenous acetylcholine and tachykinins; the latter act, in turn, on postjunctional tachykinin NK1/NK2 receptors. The role of the NK3 receptor as prejunctional mediator of the excitatory transmission operated by tachykinins is discussed.