BACKGROUND Airway sensory nerves have the capacity to release neuromediators such as substance P and nitric oxide to control airway functions. The aim of the present study was to investigate substance P and neuronal nitric oxide synthase (NOS-1) expression in airway-specific sensory neurons. METHODS Airway-projecting neurons in the jugular-nodose ganglia were investigated for NOS-1 and substance P expression by neuronal tracing and double-labelling immunoreactivity. RESULTS Of the Fast blue labelled neurons, 14.6+/-1.8% (mean+/-S.E.M.) were immunoreactive only for NOS-1, 3.0+/-0.3% for NOS-1 and substance P, 2.7+/-0.3% only for substance P, and 79.7+/-1.7% of the labelled neurons were nonimmunoreactive for substance P or NOS-1 but were partly positive for I-B4-lectin-binding. Fast blue labelled NOS and/or substance P-positive neurons were small to medium sized (<20 microm). CONCLUSIONS Based on the expression of substance P and nitric oxide synthase in airway neurons, the present study suggests that there may be substance P and NO biosynthesis and release following a peripheral activation of the afferents, there could be a triggering of substance P and NO-mediated phenomena, including those related to airway inflammation, such as plasma extravasation and vasodilatation.