BACKGROUND: The aging process may be induced, at least in part, by reactive oxygen species(ROS). It has been though that the lung could be a good source of ROS because it has a high oxygen tension. In the present study, we invetigated the inducibility of the first and last lines against oxidative stress, superoxide dismutases (Cu/Zn-SOD and Mn-SOD) as a scavenger of O2- and metallothionein(MT) as a scavenger of OH·, respectively, in mouse lungs with age. METHODS: Oxidative stress was induced by paraquat, an intracellular superoxide generator, at 1, 4, 8, and 12 months of age and then SODs and MT mRNAs were determined by RT-PCR method. RESULTS: The steady-state level of Mn-SOD mRNA increased from 1 to 8 months but decreased thereafter. However, Mn-SOD mRNA was not induced by paraquat after 1 month. On the other hand, there was no change in the steady-state level of Cu/Zn/-SOD mRNA, which decrease abruptly at 12 months of age. Additionally, Cu/Zn/-SOD mRNA was not induced by paraquat at any age. There was no change in the steady-state level of MT mRNA with age whereas its inducibility by paraquat was intact at all ages. CONCLUSION: These results indicate that lack of induction of SODs with age may be one of the causative factors in the aging process while induction of MT may play an important role in the defense against oxidative stress. It is therefore implicated that the tissue antioxidant/proxidant balance could be one determinants of meal life span.