To investigate the involvement of oxygen radicals in the development of asthma, we examined the time course of changes in the expression of superoxide dismutases (SODs) both at mRNA and protein levels in the rat model of allergic asthma. We then examined the effects of recombinant-human SOD (r-hSOD) on these expressions and on the late asthmatic response (LAR). 1) In situ hybridization histochemistry and immunocytochemistry revealed that non-sensitized and sensitized rats before challenge had a very low level of manganese SOD (MnDOS) in the bronchial epithelial cells, although they showed a significant level of copper-zinc SOD (CuZnSOD). 2) All of the animals displayed LAR within 7 hours after the challenge, when they showed dramatic induction of MnSOD, but not of CuZnSOD, in the epithelial cells. 3) Treatment with r-hSOD almost completely suppressed LAR, with abolishment of MnSOD induction. This study suggests that the oxygen radical plays an important role in the inflammatory state of bronchial asthma, during which some cytokines induce the expression of MnSOD in the lung.