Ozone, a common outdoor air pollutant, is a highly reactive gas and a major component of smog. A public health concern is that prolonged exposure to ozone might damage the airways and contribute to the development of noncancerous respiratory diseases. To examine this issue, the Health Effects Institute collaborated with the NTP to provide HEI-funded investigators access to animals that underwent the same rigorously controlled ozone exposure and quality assurance processes along with the animals used for NTP studies. One of the NTP/HEI investigator groups, Dr. Kent Pinkerton and colleagues at University of California at Davis, conducted detailed analyses of ozone\'s effects on the cellular structure of the airways and lungs. They observed cellular changes in the centriacinar region (the junction of the conducting airways and the gas exchange region of the lung) of rats exposed to 0.5 or 1.0 ppm ozone. The investigators hypothesized that these changes may protect the centriacinar region from injury. However, because measurements were made only after 20 months of exposure in the NTP/HEI collaboration, the investigators could not determine when these changes occurred, whether changes earlier in the exposure period differed from those after 20 months of exposure, or whether aging affected the results. In this follow-up study, Dr. Pinkerton and colleagues exposed male rats to 0, 0.12, or 1.0 ppm ozone for 2 or 3 months under conditions that closely replicated the original NTP exposure protocol. They conducted sophisticated structural and chemical analyses on airway tissues using techniques similar to those developed for their NTP/HEI study. They compared the results of this study with those obtained from male rats exposed to the same levels of ozone for 20 months.