Rats were exposed for 12 months, 13 hr/day, 7 days/week to 0.06 ppm O3; Monday through Friday, the animals received a 9 hr spike reaching a maximum concentration of 0.25 ppm. An increase in the rate of lung nitrogen washout was observed in the O3-exposed rats. Residual volume and total lung capacity were reduced. Glutathione peroxidase and reductase activities were increased but pulmonary superoxide dismutase was unchanged. Alpha tocopherol levels were decreased in lung lavage supernatant and unchanged in lavaged cells, however, ascorbic acid and lavage fluid protein was increased. Immunological changes were not observed. Thus, chronic exposure to O3 caused (1) functional lung changes indicative of a stiffer lung; (2) biochemical changes suggestive of increased antioxidant metabolism; and (3) no observable immunological changes.