1. The levels of active oxygen formed and thiobarbituric acid reactive substances (TBARS) in pulmonary nuclei of A/J strain were higher than those in the nuclei of ddY strain, while nuclear alpha-tocopherol level in the lungs of the former strain was lower than the the level in the latter strain. 2. Unsaturation index (UI) of nuclear fatty acids and oxidative damage to DNA in the lungs of A/J strain were higher than those in the lungs of ddY strain. 3. These results suggest that the capacity of nuclear active oxygen formation, nuclear alpha-tocopherol level and UI of nuclear fatty acids in the lungs can be main factors to regulate oxidative stress toward the pulmonary nuclei in the two strains of mice.