To study the effect of ozone in a chronically damaged lung. we used a bleomycin (BLM) induced pulmonary fibrosis model. Both endotracheal instillation of BLM and O-3 exposure both produce lung inflammation and fibrosis. Oxidative stress would be a common mechanism of damage for both BLM and O-3. Our aim was to assess lung injury induced by 5 and 60 days of intermittent exposure to 0.25 ppm O-3 in rats with bleomycin-induced pulmonary fibrosis. Thirty-day-old Sprague Dawley rats were endotracheally instilled with BLM (IU/100 g body weight) and, 30 days later, exposed to 0.25 ppm O-3 (0.25 ppm 4 h per day, 5 days a week). Histopatology controls were instilled with saline and breathing room air. Histopathological evaluation of lungs was done 5 and 60 days after O-3 exposure. BLM-induced lung damage did not change after 60 days of days of O-3 exposure. Five days of O-3 exposure increased the mean score of BLM-induced pulmonary suggesting that a short-term exposure to O-3 in a previously damaged lung might be a risk factor for developing further lung injury.