The concentration of cytochrome P-450 and activities of the microsomal enzymes aryl hydrocarbon hydroxylase and ethylmorphine demethylase were measured in hepatic tissue obtained at biopsy from 69 patients. Antipyrine half-life (AP t1/2) was measured simultaneously as an in vivo marker of drug metabolism. Values for each index of the drug-metabolizing system varied greatly, but the mean values in groups of patients with mild hepatitis or inactive cirrhosis did not differ significantly from those of controls. Hepatic cytochrome P-450 content and aryl hydrocarbon hydroxylase activity were lower in patients with severe hepatitis or active cirrhosis than in controls, but ethylmorphine demethylase activity was unchanged in the patients. Drug ingestion was associated with enhancement of drug-metabolizing enzymes in all patients but those with severe liver disease; ethylmorphine demethylase activity was enhanced proportionately more than aryl hydrocarbon hydroxylase activity or cytochrome P-450 concentration. The observation that aryl hydrocarbon hydroxylase and ethylmorphine demethylase activities are influenced to a different extent by liver disease and also by drug ingestion indicates functional heterogeneity of the hepatic microsomal drug-metabolizing system in man. Correlations between t1/2 and hepatic drug oxidases were weak, even when allowance was made for variation in liver size. Thus, the rate of drug metabolism in vivo assessed by measuring AP t1/2 does not appear to be closely related to the activity of some hepatic drug-metabolizing enzymes.