Treatment of rats with 3-methylcholanthrene increased Vmax of the high-affinity component of 7-ethoxycoumarin O-deethylase activity 60-fold. There was also an increase in Vmax of the low-affinity component. Treatment with phenobarbitone increased Vmax of the high-affinity component six-fold whilst not affecting the Km of this component. Modest changes were also observed in the kinetics of the low-affinity component. Following treatment with 3-methylcholanthrene, the sensitivity of both the high- and low-affinity components of activity to inhibition by alpha-naphthoflavone was considerably increased, with the IC50 decreasing from greater than 250 microM to less than 10 microM in both instances. Following treatment with phenobarbitone, the sensitivity of the low-affinity component to inhibition by metyrapone was considerably increased, with the IC50 decreasing from greater than 1000 microM to 96 microM. There was also a modest, but significant, increase in sensitivity of the high-affinity component to metyrapone. These results indicate that both components of 7-ethoxycoumarin O-deethylase activity are catalysed by more than one form of cytochrome P-450.