Airway nitric oxide (NO) homoeostasis is influenced by chemical and mechanical stimuli in humans; airway epithelium, which is an important site of NO production, is sensitive to osmotic challenge. The effect of inhaled hypotonic solutions on exhaled NO (eNO) is not known. In this study we evaluated the effect of ultrasonically nebulized distilled water (UNDW), a hypotonic indirect stimulus, on eNO levels. A total of 10 non-smoking healthy subjects were enrolled in the study. eNO was detected by chemiluminescence, and specific airway conductance (s G aw) was measured by plethysmography. Bronchial challenges with UNDW and with an isotonic solution were performed according to a double-blind experimental design. Baseline levels of eNO were 28.1±14.7p.p.b. UNDW did not cause any significant change in s G aw (from 0.190±0.029 to 0.181±0.036cmH 2 Oċs -1 ). With respect to baseline values, the eNO concentration decreased significantly after inhalation of 8 or 16ml of UNDW (from 26.0±13.1 to 17.2±8.5 and 16.6±7.7p.p.b. respectively; P < 0.001, n = 10). After bronchial challenge with UNDW, eNO was significantly reduced in comparison with after inhalation of the isotonic solution. In five subjects, pretreatment with N G -nitro- l -arginine methyl ester ( l -NAME), an inhibitor NO synthesis, decreased NO levels from 21.7±8.5 to 10.0±3.3p.p.b. Subsequent inhalation of 16ml of UNDW did not cause any further decrease in NO levels (10.1±3.7p.p.b.; not significant compared with l -NAME). We conclude that inhalation of aqueous solutions decreases eNO levels in healthy subjects, and that this effect is not associated with any significant change in airway calibre. The UNDW-induced decrease in eNO is not enhanced by pretreatment with the NO synthase inhibitor l -NAME, suggesting that inhaled solutions may interfere with the airway NO pathway in humans.