11-beta-Hydroxysteroid dehydrogenase (11-beta-OHSD) protects non-specific renal mineralocorticoid receptors from exposure to circulating glucocorticoid in vivo by catalyzing the conversion of corticosterone to inactive 11-dehydrocorticosterone. Although 11-beta-OHSD bioactivity and aldosterone binding sites are found in distal tubular cells, mineralocorticoid receptor and 11-beta-OHSD immunoreactivities are not colocalized. However, there are several kidney isoforms of 11-beta-OHSD, not all of which may be immunoreactive, whereas only a single mRNA species has been described. Using in situ hybridization we found 11-beta-OHSD mRNA is highly expressed in all renal tubular epithelia in the rat. It is therefore likely that 11-beta-OHSD is colocalized with mineralocorticoid receptors in distal tubular cells.