This work was done to clarify the relation between the change of lipid peroxidation and the protective systems in lungs after NO2exposure. JCL:Wistar 8‐wk‐old male rats were exposed continuously to 10 ppm NO2for 2 wk. Lipid peroxidation, measured by ethane exhalation in the breath of the rats and by the reaction of thiobarbituric acid with lung homogenates, increased to a maximum at 3 d after a decline at 1 d, and then returned to the initial level (of d 0).