The role of the NK1receptor in airway contraction induced by electrical field stimulation (EFS) was evaluated by comparing the response in NK1receptor knockout mice (NK1R−/−) with that of NK1receptor wild-type controls (WT).A frequency/response curve on tracheas from NK1R−/−mice and NK1R WT littermates was constructed. After incubation with [3H]choline, [3H]acetylcholine release upon EFS was measured by high-performance liquid chromatography and liquid scintillation counting. The effects of atropine (1×10−6 M), tetrodotoxin (1×10−6 M) and a specific NK1R antagonist (SR140333, 1×10−8 M) were studied, as well as the effects of substance P (1×10−5 M) on precontracted tracheas.Upon EFS, NK1R−/−mice had a significant lower trachea contractility than the NK1R WT animals, accompanied with less [3H]acetylcholine release. Pretreatment with atropine or tetrodotoxin abolished the EFS-induced contraction in both strains. Pretreatment with the NK1R antagonist SR140333 significantly reduced the contractility in the NK1R WT but not in the NK1R−/−mice. Substance P caused a small contraction in both NK1R WT and NK1R−/−mice. Substance P induced a relaxation in precontracted tracheas in NK1R WT but not in NK1R−/−mice.The data presented here provide direct evidence that the NK1receptor augments cholinergic neurotransmission in mouse trachea.