Rats were exposed to 0, 75, 150 or 300 ppm (1 ppm = 1 cm3/m3 = 4.35 mg/m3) m-xylene for 24 h and then killed. In the lungs, the cytochrome P-450 decreased to 45, 13 and 20% of the control value with the increasing exposure intensity and the activity of 7-ethoxycoumarin O-deethylase to 70, 27 and 14%, respectively. The activity of epoxide hydrolase increased slightly after exposures both at 150 (1.6-fold) and 300 cm3/m3 (1.4-fold), while the other measured drug-metabolizing enzyme activities showed no consistent changes. The non-protein sulfhydryl group content of the lungs was not affected. The concentrations of m-xylene in blood indicated that the solvent uptake increased in the different exposure groups more than expected, based on atmospheric concentrations alone. Morphologic studies of the lungs with scanning electron microscopy showed no apparent changes after exposure to 300 cm3/m3 or after a high oral dose (2 ml/kg/day, 3 days). Inhalation exposure to m-xylene for 5 weeks (7 h/day, 4 days/week) at a concentration of 300 ppm lowered the contents of cytochrome P-450 in rat lungs to 65% and the activity of 7-ethoxycoumarin O-deethylase to 41% without any other marked effects on the other drug-metabolizing enzymes or on the levels of non-protein sulfhydryl groups. In this study, the selective destruction of cytochrome P-450 in rat lung could be shown both after acute and subacute exposures and at concentrations low enough to warrant occupational concern.