Exposure to airway irritants during early postnatal periods may contribute to asthma during childhood especially if the initial exposure occurs at critical periods of growth and development. Asthma in children is exacerbated by air pollution that may be partly attributed to neural mechanisms. This study examines the effects of ozone on SP nerve fiber density in neonatal through adolescent rat airways. Rats were exposed to 2 ppm ozone or filtered air for 1 hr at the following postnatal days (PD) of age: PD 0-3 days, PD 4-6, PD 7-9, PD 10-12, PD 13-15, PD 21-22, and PD 28-29(n=5-6 per group). After 24 hr, lungs were removed, fixed and prepared for immunocytochemical demonstration of SP. Extrapulmonary (EP, main stem bronchi) and intrapulmonary (IP, 3-5 generations) airway were dissected and nerve fiber density (NFD) was determined for epithelium (E) and smooth muscle (SM). SP NFD in the EP-E increased threefold from about 1% to 4% from PD 0-3 through PD 13-15 and was then maintained. NFD for EP-SM, IP-SM, and IP-E was constant through the age range at about 1.0%, 0.6% and 0.05% respectively. After ozone, NFD increased twofold to about 2% in EP-SM through PD13-15 and to 1.2% in IP-SM through PD 13-15 while EP-E and IP-E were minimally affected. The findings suggest that nerve fiber responses to ozone are moot pronounced in SM through PD 15 possibly identifying a critical developmental phase of neural sensitivity during which irritant exposure may be a predisposing factor to the pathogenic mechanisms leading to asthma.