Ozone is a highly reactive gas that is a pervasive air pollutant at ground level. It is a major component of urban smog, forming when emissions from mobile and industrial sources interact with sunlight. The study of the effects of long-term ozone exposure on lung collagen, described in this report, was one of eight studies in a Collaborative Project supported by the NTP and the HEI. The others included studies of lung biochemistry, structure, and function, and one study of nasal structure and function. Dr. Jerold Last and coworkers at the University of California at Davis used biochemical methods to quantify total collagen and specific collagen cross-links in lung lobes from male and female rats exposed for twenty months to 0, 0.12, 0.50, and 1.0 ppm ozone. Dr. Jack Harkema (another investigator in the NTP/HEI Collaborative Project) studied the increases in collagen deposits at specific sites in the lungs of the same rats.