The effect of a prolonged ozone exposure (1.6 mg ozone/m3; 7 d; 24 h/d) on pulmonary cytochrome P-450-dependent xenobiotic metabolism was studied both in whole rat lung as well as in isolated bronchiolar Clara cell preparations. Ozone exposure was demonstrated to result in significant quantitative but also qualitative changes. All components of the pulmonary microsomal electron transport system appeared to be significantly increased in the lungs of exposed animals both per lung and per gram lung, although increases were no longer observed when expressed per milligram microsomal lung protein. Remarkably, it was demonstrated that the increases in the components of the pulmonary cytochrome P-450 system were not accompanied by a concomitant increase in all cytochrome P-450-dependent substrate conversions. In whole-lung microsomes ethoxycoumarin O-deethylase and ethoxyresorufin O-deethylase activities were unchanged or even significantly reduced when expressed per lung, per gram lung, per milligram microsomal protein, or per picomole cytochrome P-450. In contrast to these observations, pentoxyresorufin O-dealkylation appeared to be significantly increased upon ozone exposure when expressed per lung, per gram lung, and even per picomole cytochrome P-450. Clara cell populations isolated from ozone exposed rats showed a comparable qualitative shift in cytochrome P-450-dependent substrate conversion characteristics. On a cellular basis, ozone exposure resulted in a significant reduction of ethoxycoumarin and ethoxyresorufin O-deethylation and did not affect pentoxyresorufin O-dealkylase activity. Additional experiments, in which ozone-mediated inactivation of microsomal cytochrome P-450-dependent substrate metabolism was studied in vitro, demonstrated that the qualitative changes observed after in vivo exposure cannot be ascribed to a disproportional inactivation of different cytochrome P-450 isoenzymes. Based on these results and on lung morphometrics and cell isolation data presented, the observed effects should rather be ascribed to (1) the proliferation of cytochrome P-450 containing cell populations and (2) intrinsic cellular biochemical changes. The quantitative and qualitative ozone-induced changes in pulmonary cytochrome P-450-linked enzyme characteristics in whole lung and within specific lung cells, as demonstrated in the present study, may be expected to have important implications for the toxicity of xenobiotics whose (de)toxification depends on pulmonary cytochrome P-450-dependent metabolism.