The present study investigated whether there are changes in nasal peak inspiratory flow (NPIF) during hypobaric hypoxia under controlled environmental conditions.During operation Everest III (COMEX '97), eight subjects ascended to a simulated altitude of 8,848 m in a hypobaric chamber. NPIF was recorded at simulated altitudes of 0 m, 5,000 m and 8,000 m. Oral peak inspiratory and expiratory flow (OPIF, OPEF) were also measured. Ambient air temperature and humidity were controlled.NPIF increased by a mean±sd of 16±12% from sea level to 8,000 m, whereas OPIF increased by 47±14%. NPIF rose by 0.085±0.03 L·s−1per kilometre of ascent (p<0.05), significantly less than the rise in OPIF and OPEF of 0.35±0.10 and 0.33±0.04 L·s−1per kilometre (p<0.0005).Nasal peak inspiratory flow rises with ascent to altitude. The rise in nasal peak inspiratory flow with altitude was far less than oral peak inspiratory flow and less than the predicted rise according to changes in air density. This suggests flow limitation at the nose, and occurs under controlled environmental conditions, refuting the hypothesis that nasal blockage at altitude is due to the inhalation of cold, dry air. Further work is needed to determine if nasal blockage limits activity at altitude.