The effects of chronic inhalation of diesel exhaust and coal dust on lung and liver microsomal enzyme activities were investigated in 6 week old male Fischer-344-rats. Animals were exposed for 7 hours per day, 5 days per week, for 24 months to: 2 milligrams (mg) diesel exhaust per cubic meter (m3) air; 5mg/m3 coal dust; or 3mg/m3 diesel exhaust and coal dust. Rats were sacrificed at 3, 6, and 24 months after the start of exposure, and microsomes were prepared from lungs and livers. Assays were conducted for: benzo(a)pyrene (BaP)/hydroxylase; 7-ethoxycoumarin deethylase; protein; and cytochrome-P-450. Age related changes in enzyme activities were observed, but they were not altered by the exposures. When the data was adjusted for age, only one difference was observed: lung BaP/hydroxylase activity in rats exposed to both diesel exhaust and coal dust was lower than that in animals exposed to coal dust alone. Neither value, however, differed significantly from the filtered air controls, and no differences were observed in the other lung and liver activities. The authors conclude that exposure of rats to coal dust or diesel exhaust has no effect on selected lung and liver cytochrome-P-450 activities under the conditions of the experiment.