The metabolism of the synthetic glucocorticoid dexamethasone in human liver microsomal incubations has been studied. Metabolites were analyzed by radiometric high-performance liquid chromatography and were identified by liquid-chromatography-mass spectrometry; in addition, the major metabolite 6beta-hydroxydexamethasone was identified by cochromatography with a chemically synthesized standard. A total of 17 human livers were used in this study and the following metabolites were identified: 6beta-hydroxydexamethasone, 6 alpha-hydroxydexamethasone, 6-hydroxy-9 alpha-fluoro-androsta-1,4-diene-11 beta-hydroxy-16 alpha-methyl-3,17-dione (6-hydroxy-9 alpha-F-A) and 9 alpha-fluoro-androsta-1,4-diene-11 beta-hydroxy-16 alpha-methyl-3,17-dione (9 alpha-F-A). Dexamethasone underwent side-chain cleavage to form 9 alpha-F-A. This metabolite was then a substrate for 6-hydroxylation. There was considerable interindividual variability in metabolic profiles. Mean (+/-S.D.) K(m) values for 6 beta- and 6 alpha-hydroxydexamethasone formation were 23.2 +/- 3.8 and 25.6 +/- 1.6 microM (n = 4), respectively. The corresponding V max values were 14.3 +/- 9.9 and 4.6 +/- 3.1 pmol x min(-1) mg protein (-1). Ketoconazole (3 microM) completely inhibited 6 alpha- and 6 beta-hydroxylation, indicating that formation of both metabolites was catalyzed by CYP3A4. This was confirmed in studies of correlations between the rate of metabolite formation and the relative expression of CYP3A4: r = 0.74 for 6 beta-hydroxydexamethasone, P = .003; r = 0.70 for 6 alpha-hydroxydexamethasone, P = .006. In addition to ketoconazole, both ellipticine and gestodene caused marked inhibition of 6-hydroxylation. Ellipticine is clearly not a selective CYP1A inhibitor as has been stated previously. However, furafylline (CYP1A inhibitor), tolbutamide (CYP2C substrate), and sulfaphenazole (CYP2C inhibitor) were essentially noninhibitory. The relatively simple metabolic profile of dexamethasone compared to other steroids may point to this being a potentially useful in vivo probe for CYP3A4 in humans.