Sixteen healthy non smoking subjects (7 female), 21 to 49 years old, were exposed in a climate chamber to either clean air or 300 ppb ozone on 4 days for 5 hrs each day. Before each exposure the subjects had been pre-treated with either oxidants (fish oil) or antioxidants (multivitamins). The study design was double blind cross-over with randomized allocation to the exposure regime. Full and partial flow volume curves were recorded in the morning and before and during a histamine provocation at the end of the day. Nasal cavity volume and inflammatory markers in nasal lavage fluid were also measured. Compared with air, ozone exposure decreased PEF, FEV₁ and FVC with no significant effect from the pre-treatment regimens. Ozone decreased the ratio of maximal to partial flow at 40% FVC by 0.08±0.03(mean±SE, analysis of variance: p = 0.018) and at 30 % FVC by 0.10±0.05 (p=0.070). Ozone exposure did not significantly increase bronchial responsiveness, but after treatment with fish oil partial flows decreased more than after vitamins during the histamine test, without changing the M/P flow ratio. The decreased effect of a deep inhalation after ozone exposure can be explained by changes in airway hysteresis relative to parenchymal hysteresis, due either to ozone induced airway inflammation or to less deep inspiration after ozone, not significantly influenced by multivitamins or fish oil.