We examined the effect of polluted roadside air on the pathogenesis of chronic respiratory diseases, by exposing rats directly to roadside air in Kawasaki City. Five-week-old Wistar rats were assigned to be exposed to either roadside air (containing 55.7 ppb NO2 and 62.7 microg/m(3) suspended particulate matter [SPM]; roadside-air group) or filtered air (containing 5.1 ppb NO2 and 14.3 microg/m(3) SPM; filtered-air group), prepared by eliminating dust and emission gas from the polluted roadside air, for a period of 60 consecutive wk. The morphological changes over time in the respiratory tissue of these animals were observed by light microscopy and electron microscopy at 24, 48, and 60 wk of exposure. In addition to the general microscopic findings, other features observed for included the proliferation of goblet cells producing mucus in the airways, acidification of neutral mucous granules in these goblet cells, the behavior of inflammatory cells such as alveolar macrophages, mast cells, and plasma cells, and the number of alveolar holes, known to be a primary indicator of early alveolar destruction. Focal anthracosis, acidified mucus in the goblet cells, and infiltration to the submucosa by inflammatory cells were observed in the lungs of the animals after 60 wk of exposure to polluted roadside air. However, these inflammatory changes were weak. No remarkable differences in the number of alveolar holes were noted between the two experimental groups. These findings suggest that the effects of roadside air on the respiratory tissue in rats may not be as severe as would be expected under these experimental conditions.