Electron microscopy morphometry has been used to study the effects of cumulative exposure of low levels of inhaled O3 on lung proximal alveolar tissue. Six-week-old Fisher 344 rats were exposed to O3 in two different subchronic low-level exposure patterns. The first was a 12 hr/day exposure for 6 weeks and included two O3 concentrations, 0.12 and 0.25 ppm. The second consisted of an exposure profile having a background level of 0.06 ppm with an exposure peak 5 days each week that went from 0.12 to 0.25 ppm and back to 0.12 ppm over a 9-hr period. Rats given the second exposure pattern were exposed for either 3 or 13 weeks. Changes in the volumes of alveolar epithelium were found to be consistent and reproducible markers for cell injury and/or response. Results from the first study indicated that the relative volume of the type I epithelium increased 13 and 23% over the control value (p less than 0.05) following exposures for 6 weeks to 0.12 and 0.25 ppm, respectively. The magnitude of the increases were clearly concentration related. Similarly, when a fixed exposure concentration was employed the relative volume of type I epithelium was found to increase in proportion to the exposure time. In the second exposure, increases of 9 and 33% in relative volume of type I epithelium were found respectively after 3 and 13 weeks of exposure. If the total exposure determined by the product of O3 concentration (including background) and exposure time is plotted against the relative volume of type I epithelium from both the 0.12 ppm (60.5 ppm-hr) and 0.25 ppm (126 ppm-hr) exposures and the 3-week (45.3 ppm-hrs) and 13-week (196.2 ppm-hr) exposures, a linear relationship between increases in type I cell volume and the concentration X time product is observed. The coefficient of correlation (r2) for the linear regression of the animal means is 0.72. Changes in the volume of Type II epithelial cell also correlate with the concentration X time product (r2 = 0.66). This suggests that epithelial cell reactions to low-level subchronic exposure of O3 are directly related to the cumulative oxidant concentration. The pattern of exposure did not appear to affect the resulting degree of injury. Furthermore, a low level of background exposure may contribute to the epithelial cell injuries.