One day old or 6 week old male rats were exposed to either 0.25 ppm O3 12 hr/day or to continuous room air for 6 weeks. An additional group of 6 week old rats was exposed to 0.12 ppm of O3 for the same time period. In the animals exposed to 0.25 ppm O3 from 1 day of age (juvenile animals), the number of type I epithelial cells doubled, their mean surface area decreased 38%, and their mean thickness increased 24%. The number of alveolar macrophages doubled. Adult animals exposed to 0.25 and 0.12 ppm O3 showed similar patterns of changes in the epithelium of the proximal alveolar region. Compared to the juvenile animals, the adult 0.25 ppm O3 exposed animals showed more reaction in the interstitium with a doubling of interstitial macrophages, suggesting a mild inflammatory stimulus in the interstitium. Animals exposed to 0.12 ppm O3 showed smaller, but statistically significant changes in the alveolar type I epithelium, suggesting a relatively linear concentration response relationship. These results suggest that low concentrations of O3 cause a chronic epithelial injury in the proximal alveolar region of both juvenile and adult rats and that the extent of these changes occurs in a concentration-dependent manner, even at concentrations at low as 0.12 ppm.