Fischer 344 rats were exposed to ozone by inhalation to identify sensitive indices of acute exposure. 5-Aminosalicylic acid (5-ASA) hydroxylation in bronchoalveolar lavage (BAL), an indicator of hydroxyl radical (*OH) formation, and lipid oxidation in various regions of airways, representing oxidative stress, were measured to verify whether they can function as markers of exposure. BAL cells and supernatants taken from rats that received saline or 5-ASA (ip, 50 mg/kg) prior to ozone exposure (0, 0.4, or 0.8 ppm for 4 h) were analyzed for products of lipid oxidation. *OH formation was assessed by analysis of the BAL supernatant for 5-aminotetrahydroxybenzoic acid (5-ATHBA), a hydroxylation product of 5-ASA. The tetrahydroxy derivative of 5-ASA was higher in the BAL of ozone-treated rats than in air controls, reaching significance (p <. 05) at 0.8 ppm of ozone, The products of lipid oxidation propanal and hexanal were higher in BAL cells taken from rats exposed to ozone, reaching significance (p <. 05) at a 0.8 ppm ozone level, compared to air control animals, irrespective of whether they received saline or 5-ASA prior to ozone exposure. Increases in cholesterol levels were also seen in BAL cells after rats were exposed to ozone. However, there were no significant dose-related changes in the lipid oxidation products in BAL supernatants after exposure to ozone. Lipid oxidation products in BAL cells and 5-ATHBA in lavage exhibited the potential to serve as markers of ozone exposure. This work was supported by Health Canada (#4320105) and Toxic Substances Research Initiatives (TSRI #60).