Cells of the terminal bronchioles are particularly susceptible to the effects of inhalation of low levels of O3. One-day-old (juvenile) or 6 week old (adult) rats were exposed to 0.25 ppm O3 12 hours/day for 6 weeks. Morphometric analysis of perpendicular cross sections of terminal bronchioles demonstrated that exposure to O3 produced alterations in the surface characteristics of ciliated and non-ciliated (Clara) cells in both groups of rats. There were significant losses (20-30 percent) of the surface area contributed by cilia and the luminal surface of Clara cells was decreased by 16 percent to 25 percent. O3 exposure produced significant decreases in the number of brush cells per sq mm of terminal bronchiolar basement membrane. The results of the study indicate that the normal structure of terminal bronchiolar epithelial cells is significantly altered by inhalation of 0.25 ppm O3. No statistically significant interactions between the effects of O3 and animal age at the beginning of the exposure were found. (Copyright (c) 1988 by Hemisphere Publishing Corporation.)