The effect of repeated daily exposure to a low concentration of ozone was studied in healthy adult males to answer 2 questions: (1) Is there a cumulative effect of exposure to ozone from day to day, causing an exacerbation of changes in pulmonary function? (2) Does the subject develop resistance or adaptatin to the respiratory effects of ozone? The subjects were exposed for 2 h in an environmental chamber at 35 degrees C and 45% relative humidity to filtered air on day 1, to ozone on days 2 through 4, and to filtered air on day 5. Three groups of subjects were used, each exposed to a different concentration of ozone: group 1 (n = 10), 0.20 ppm of ozone; group 2 (n = 10), 0.35 ppm of ozone; group 3 (n = 8), 0.50 ppm of ozone. There were no acute or cumulative effects of repeated exposure to 0.20 ppm of ozone. With exposure to 0.35 ppm of ozone, there were decrements in forced expiratory parameters on day 2. These changes were greater on day 3, but wre largely absent on days 4 and 5. In group 3, there were marked effects of ozone on pulmonary function on day 2, but these changes were greatest on day 3. Significant effects (although of lesser magnitude) were also observed on day 4. The forced expiratory volume in 1 s decreased 8.7% on day 2, 16.5% on day 3, and 3.6% on day 4. The subjects claimed the most discomfort on day 3; many noted marked improvements on day 4, the third consecutive day of exposure to ozone. Two additional subjects were exposed to 0.50 ppm of ozone for 4 consecutive days. Although effects of ozone on pulmonary function were observed on the first 2 days of exposure, few effects were seen on the third day, and no effect was observed on the fourth day. We conclude that there is some short-term cumulative effect of exposure to concentrations of ozone above the threshold for producing acute respiratory effects. This is followed by a period of resistance or adaptation in which there is a marked lessening of the effect of ozone on pulmonary function and on the subjective feelings of discomfort associated with exposure to ozone.