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It has been demonstrated that the formation of the hydrophilic metabolites of dexamethasone, 6alpha- and 6beta-hydroxydexamethasone, correlated with cytochrome P450 (CYP) 3A4 enzyme levels. So, the 6beta-hydroxydexamethasone/dexamethasone urinary ratio could be a specific marker for human CYP3A4 activity. We have developed a sensitive and specific high-performance liquid chromatographic method for the simultaneous quantification of urinary free dexamethasone and 6beta-hydroxydexamethasone using 6alpha-methylprednisolone as internal standard. This method involved a solid phase extraction of the three compounds from urine using Oasis HLB Waters cartridges with an elution solvent of ethyl acetate (2 ml) followed by diethyl ether (1 ml). Separation of the three analytes was achieved within 24 min using a reversed-phase Nova-Pak C-18 analytical column (4 mum, 300 mm x 3.9 mm i.d.). An ultraviolet detector operated at 245 nm was used with a linear response observed from 10 to 100 ng/ml for dexamethasone and from 25 to 1000 ng/ml for 6beta-hydroxydexamethasone. Obtained from the method validation, inter-assay precision was below 15% and accuracy ranged from 95.7 to 110%. The extraction efficiency of the assay was approximately of 99% and was constant across the calibration range. The lower limit of quantitation was 10 ng/ml for dexamethasone and 25 ng/ml for 6beta-hydroxydexamethasone; at these levels, precision was below 16% and accuracy was 99-109%. This method was applied to in vivo measure of the CYP3A4 activity. (C) 2004 Elsevier B.V. All rights reserved.