Human plasma contains similar free concentrations of the active steroid cortisol and its inactive metabolite cortisone. Interconversion of cortisol and cortisone (or corticosterone and 11-dehydrocorticosterone in rats) has been recognized since the 1950s. Until recently it was thought that separate NADP/NADPH-dependent enzymes were responsible: 11β-dehydrogenase to convert cortisol to cortisone and 11β-reductase for the reverse reaction (Abramowitz, Branchaud & Murphy, 1982; Lakshmi & Monder, 1985). It is now known that both activities can be expressed from the same cDNA clone (Agarwal, Monder, Eckstein & White, 1989; Agarwal, TusieLuna, Monder & White, 1990), suggesting that a single 11β-hydroxysteroid dehydrogenase protein (11β-OHSD) catalyses both reactions. Recently, interest in the enzyme was stimulated by observations in patients with congenital 11β-dehydrogenase deficiency (the syndrome of apparent mineralocorticoid excess) (Ulick, Levine, Gunczler et al. 1979; Stewart, Corrie, Shackleton & Edwards, 1988) and in volunteers given the 11β-dehydrogenase inhibitors liquorice (Stewart, Valentino, Wallace et al