In the present study, changes of lipid peroxides, phospholipids and antioxidant levels in lungs of 4 animal species exposed to the combined gases of NO2 and O3 were compared. Male mice, hamsters, rats and guinea pigs were used. Lipid peroxides were increased significantly in the lungs of mice and guinea pigs exposed to the combined gases, but not in hamsters and rats. Changes of alpha-tocopherol (VE) contents were slight. On the other hand, non-protein sulfhydryl (NPSH) contents were increased strikingly, especially in hamsters, but were not increased in guinea pigs. Phosphatidylcholine (PC) contents were increased and phosphatidylethanolamine (PE) contents were decreased by the exposure to the combined gases, with the order guinea pig greater than mouse greater than rat. In hamsters no changes were seen. The changes of fatty acid composition in guinea pigs and mice were marked, the increases of palmitate and palmitolate and the decreases of polyunsaturated fatty acid were especially characteristic. These changes in phospholipid class and fatty acid composition may be a "a kind of adaptation phenomenon" to avoid further lipid peroxidation. On the other hand, the changes in hamsters and rats were small. The results show the existence of species differences in lipid peroxide formation by exposure to the combined gases of NO2 and O3. They were found to be related to the contents of antioxidants and the compositions of phospholipids and their fatty acids.