The pathogenesis of NO2‐induced respiratory disease and the effect of NO2on respiratory clearance ofs1Cr2Oiwere studied. Young adult female mice were exposed to a 51Cr2O3aerosol, followed by a daily exposure to either 30 ppm NO2for 2 wk or 60 ppm NO2for 2 wk or a single exposure to 170 ppm NO2. Exposure to 30 ppm NO2had a minimal histopathologic effect on respiratory tract tissue. Exposure to 60 ppm NO2produced marked histopathologic effects, which were subsequently resolved. Exposure to 170 ppm NO2produced permanent histopathologic lesions.