Male Wistar rats were exposed to 0.2 and 0.4 ppm O3 for 4, 8, and 12 weeks and to 0.1 and 0.2 ppm O3 for 4 weeks to examine the effects of prolonged exposure to O3 on the xenobiotic metabolizing systems in the lung. Exposures to 0.2 and 0.4 ppm O3 caused a significant increase in the NADPH-cytochrome P-450 reductase activity and cytochrome P-450 content in a dose-dependent manner during 4-12 weeks, whereas NADH-cytochrome b5 reductase was not altered. After 12 weeks of exposure to 0.4 ppm O3, NADPH-cytochrome P-450 reductase and cytochrome P-450 reached a maximum showing 138 (P less than 0.001) and 221% (P less than 0.001) those of the control levels, respectively. At 0.1 ppm O3, the cytochrome P-450 content still increased significantly to 152% that of the control on the fourth week. In parallel to the increment of cytochrome P-450 benzo(a)pyrene hydroxylase and 7-ethoxycoumarin O-deethylase increased significantly during 4-12 weeks of exposures to 0.2 and 0.4 ppm O3. After a 4-week exposure to 0.1 and 0.2 ppm O3, benzphetamine N-demethylase increased to the largest degree among the enzymes metabolizing four kinds of xenobiotics examined. 7-Ethoxycoumarin O-deethylase also increased significantly but of smaller magnitude, whereas coumarin hydroxylase was not affected. These results show that prolonged exposures to 0.1-0.4 ppm O3 persistently enhance pulmonary cytochrome P-450 systems. It is also suggested that some isozyme(s) of pulmonary cytochrome P-450, especially that catalyzing benzphetamine N-demethylation, are preferentially increased by exposure to low levels of O3.