Nitric oxide (NO) is produced in large amounts in the noses of normal individuals. We have measured NO by chemiluminescence in the noses and exhaled air of subjects with symptomatic allergic rhinitis, some of whom had concomitant asthma, during the pollen season and compared this with values measured in normal subjects and in patients treated with nasal and/or inhaled glucocorticoids. We found that nasal levels of NO were significantly (p < 0.001) elevated in patients with untreated rhinitis (1527 +/- 87 ppb, n = 12) compared with normal individuals (996 +/- 39 ppb, n = 46) or subjects treated with nasal steroids (681 +/- 34 ppb, n = 10), whereas exhaled NO in patients with untreated rhinitis was similar to that in normal subjects (10 +/- 2 ppb vs 7 +/- 0.6 ppb, respectively). In five subjects who were nasally challenged with allergen, there was a significant decrease in nasal NO 1 hour after challenge, and this was significantly correlated with increased rhinitis symptoms. In patients with rhinitis and concomitant asthma, nasal NO was also significantly elevated (1441 +/- 76 ppb, n = 16) but not when they were treated with nasal or inhaled steroids; whereas exhaled NO was elevated in untreated patients and in patients treated with nasal, but not inhaled, steroids. Our data suggest that the increase in exhaled NO in patients with allergic rhinitis is likely to be due to increased local production, caused by long-term exposure to allergen, which is suppressed by locally administered steroids. Measurement of nasal NO may be useful to study the inflammatory response in rhinitis and its response to antiinflammatory treatments.