The changes in lipid peroxides (LP) content and antioxidant enzyme activities were investigated on airway surface during the aggravating process of bronchitis induced by SO2 exposure in rats. LP content in broncho-alveolar lavage fluid (BALF) has gradually increased from 3 weeks after starting of SO2 exposure. Whereas, the activities of antioxidant enzymes, superoxide dismutase and glutathione peroxidase, increased at 1 week and then gradually reduced from 3 weeks. The ratio of LP to each antioxidant enzyme activity in BALF of the exposed rats was higher than that of normal rats. Morphological changes of the lung, a decrease of PaO2 and an increase of PaCO2 of blood depended on the increase of LP on airway surface. These findings indicate that LP may be involved in the development of bronchitis.