We have used in vitro techniques to study the metabolism of dexamethasone. Tissue slices, homogenates and microsomal fractions of various mammalian organs from rats and humans have been used. We focused particularly on the question of whether or not dexamethasone (Dexa) is oxidized at the C11-OH group by 11β-hydroxysteroid-dehydrogenase. High activities of this enzyme system for Dexa were localized in renal cortex and rectum. Material from both human and murine liver was ineffective. The main metabolite formed from Dexa in renal and intestinal systems was identified by different mass-spectrometric techniques including on line HPLC mass spectrometry as 11-dehydro-dexamethasone. This finding was corroborated by the observation that both corticosterone and glycyrrhetinic acid block the metabolic transformation of Dexa