1. We characterized the tachykinin receptor(s) mediating 'sensory-efferent' neural control of release of 35SO4-labelled macromolecules (mucus) from ferret trachea in vitro in Ussing chambers using selective tachykinin antagonists. Secretion was induced by substance P (SP), neurokinin A (NKA), capsaicin, the NK1 tachykinin receptor agonist [Sar9, Met(O2)11]substance P ([Sar9]SP), or acetylcholine (ACh), or by electrical stimulation of nerves. Antagonists used were FK888 and L-668,169, selective for the NK1 receptor, SR 48968, selective for the NK2 receptor, and FK224, a dual antagonist at NK1 and NK2 receptors. The selectivity of FK888 and SR 48968 was examined on NKA-induced contraction of ferret tracheal smooth muscle in vitro. 2. SP (1 microM) increased mucus secretion by 695% above vehicle controls. FK888 (0.1 microM-30 microM) inhibited SP-induced secretion in a dose-dependent manner, with complete inhibition at 10 microM and an IC50 of 1 microM. L-668,169 (1 microM) also completely inhibited SP-induced secretion. 3. NKA (1 microM) significantly increased mucus secretion by 271% above baseline, a response which was completely inhibited by FK888 (10 microM) or L-668,169 (microM). Secretion induced by ACh (10 microM: 317% above baseline) was not inhibited by FK888 but was inhibited by atropine. Capsaicin (10 microM)-induced secretion (456% above vehicle controls) was significantly inhibited by FK888 and by L-668,169 (111% and 103% inhibition respectively). 4. Electrical stimulation (50 V, 10 Hz, 0.5 ms, 5 min) increased mucus output above baseline (increased by 12 to 26 fold), a response blocked by tetrodotoxin (0.1 microM). FK888 (10 microM) inhibited the increase in secretion due to electrical stimulation by 47%. Atropine, propranolol and phentolamine in combination(APP) inhibited the response to electrical stimulation by 48%. The remaining NANC response, i.e. in the presence of APP, was further reduced by 66% with FK888. FK224 (10 microM) inhibited neurally evoked secretion by 73%. SR 48968 (0.1 fLM) had no effect on electrically-stimulated or [Sar9]SP-induced secretion.5. NKA (10nM- 1O microM: in the presence of DMSO control vehicle) induced tracheal smooth muscle contraction in a concentration-dependent manner with a maximal contraction of 30% of the maximal response to ACh (10 mM) and an ECm of 0.3 JAM. SR 48968 (0.1 microM in DMSO) inhibited the NKA induced contraction whereas FK888 did not. Neither antagonist had any inhibitory effect on ACh induced contraction.6. We conclude that 'sensory-efferent' neurogenic mucus secretion in ferret trachea in vitro is mediated via tachykinin NK, receptors with no involvement of NK2 receptors. Potent and selective tachykinin antagonists may have therapeutic potential in bronchial diseases such as asthma and chronic bronchitis in which neurogenic mucus hypersecretion may be aetiologically important.