The effects of ambient oxygen tension on epithelial metabolism and nitric oxide (NO) production (VNO) in the nasal airway was examined in nine healthy volunteers. Nasal VNO, oxygen consumption (VO₂) and carbon dioxide production (VCO₂) were measured during normoxia followed by gradual hypoxia from 21 to 0 % O₂ concentration. Nasal VO₂, VCO₂ and respiratory quotient (R) during normoxia were determined to be 1.19 ± 0.04 ml/min, 1.60 ± 0.04 ml/min and 1.35 ± 0.04, respectively. Hypoxia exposure to the nasal cavity significantly decreased both VCO₂ and VNO [ VCO₂: 1.60 ± 0.04 to 0.96 ± 0.03 ml/min (p < 0.01), VNO: 530 ± 15 to 336 ± 9 nl/min (p < 0.01)]. VNO was reduced commensurately with gradual decline in O₂ tension and the apparent Michaelis-Menten constant (K_m) value for oxygen was determined to be 23.0 µM. These results indicate that the nasal epithelial cells exchange O₂ and CO₂ with ambient air in the course of their metabolism, and nasal epithelial cells can synthesize NO by using ambient oxygen as a substrate. We conclude that air-borne oxygen diffuses into the epithelium where it may be utilized for either cell metabolism or NO synthesis.