Background and objectives: A large body of evidence suggests that ozone plays an important role in the development of respiratory allergy. However, the mechanisms by which ozone may interact with airway responses to allergen are not clear. The aim of this study was to determine whether ozone enhances the nasal inflammatory responses to allergen challenge in ovalbumin-sensitized rats and whether ozone has an effect on the allergic sensitization process. METHOD: Twenty-seven Wistar rats were divided into five groups : control group (group 1), group exposed to 0.3 ppm ozone for 3 days (6 hr/day) (Group 2), group challenged by allergen in ovalbumin-sensitized rats (group 3), group exposed to ozone followed by allergen challenge in ovalbumin-sensitized rats (group 4), and group exposed to 0.3 ppm ozone for 6 hours at intervals of 3 days during allergen sensitization (group 5). To induce the allergic rhinitis in groups 3, 4, and 5, rats were immunized intraperitoneally with ovalbumin-Al(OH)3 gel solution, followed by intranasal nebulization of 0.1% ovalbumin solution. Nasal lavage fluid (NLF) was obtained immediately after the last exposure and then Evans blue dye was infused via the femoral vein. The concentration of protein and total cell counts in NLF and the amount of extravasated Evans blue dye in nasal mucosa were measured. RESULTS: Ozone exposure significantly increased the concentration of protein and total cell counts in NLF and Evans blue extravasation compared to the control group. Allergen challenge in sensitized rats increased total cell counts in NLF and Evans blue extravasation although the differences between this group and the controls did not reach statistical significance. The combination of ozone exposure and allergen challenge caused significant increase in total cell counts in NLF and Evans blue extravasation compared to groups 2 and 3. However, there were no significant differences in Evans blue extravasation, concentrations of protein and total cell counts in NLF between groups 3 and 5. CONCLUSION: Ozone may enhance the nasal inflammatory responses to allergen in patients with allergic rhinitis.