Compared with normal individuals, subjects with asthma have elevated levels of expired nitric oxide (NO). These levels are hypothesized to reflect the degree of airway inflammation. Expired NO levels rise during the late phase of allergen challenge and decrease in asthmatics after steroid treatment. Isocapnic cold air hyperventilation (ISH) is believed to cause airway narrowing through noninflammatory mechanisms. We measured mixed expired NO in 10 individuals with atopic asthma who underwent both ISH challenge and allergen challenge, and compared these measurements with the change in expired NO that occurred after serial spirometry alone. We found that ambient NO levels affected mixed expired NO. Controlling for inspired NO, we found that repeated spirometry alone produced a significant fall in mixed expired NO (p < 0.01) that was maximal after 30 min (36.6 +/- 8.5% fall). After allergen and ISH challenges, expired NO was elevated relative to levels after repeated spirometry (p < 0.01 and p = 0.065, respectively). In addition, we found that prechallenge expired NO levels were significantly correlated with the magnitude of the late fall in FEV1 following allergen challenge (r = 0.80, p < 0.01). These data demonstrate that repeated spirometry results in reduced mixed expired NO and suggest that both ISH and allergen-induced bronchoconstriction share pathobiologic mechanisms that produce increases in mixed expired NO.