Groups of rats and guinea pigs with normal lungs and others with elastase-induced emphysema were exposed to NH4NO2 aerosols of 0.60 MMAD at 1 mg/cu m for 6 hr/day, 5 days/wk for 4 wks. Morphologic and morphometric studies were performed on lungs perfused with cacodylate-buffered 2% glutaraldehyde under 20 cm H2O pressure at necropsy. The tissues were studied for pathologic change by light microscopic methods, including changes in mean alveolar chord length, using scanning electron microscopy (SEM) techniques. Elastase produced emphysema to a degree quantifiable by all criteria studied; however, it apparently obscured the effects of nitrate inhalation. The NH4NO3 exposures (compared to air alone) tended to increase values for pulmonary parameters in normal animals of both species and to decrease them in elastase-treated animals. The responses to NH4NO3 were slight (P < 0.10) and were not accompanied by any detectable changes in alveolar structure. Therefore, the effects of NH4NO3, at this exposure level and duration, are regarded as biologically insignificant for rats and guinea pigs. (Copyright (c) by Academic Press, Inc.)