11 beta-Hydroxysteroid dehydrogenase (11 beta-HSD) dictates specificity for the mineralocorticoid receptor (MR) by converting the active steroid cortisol to cortisone in man (corticosterone to 11-dehydrocorticosterone in rodents), leaving aldosterone to occupy the MR. However cortisol is the principal circulating glucocorticoid in man and 11 beta-HSD, distributed in a tissue specific fashion, may represent a powerful mechanism in regulating exposure of active steroid to the glucocorticoid receptor (GR). A detailed localization study of 11 beta-HSD gene expression and activity in numerous rat tissues has been performed and compared with the presence of GR mRNA. 11 beta-HSD mRNA (1.4 kB) measured by hybridization to a cDNA derived from hepatic 11 beta-HSD, and enzyme activity, measured by percentage conversion of [3H]corticosterone to [3H]11-dehydrocorticosterone by tissue homogenate, was widespread, present in all tissues studied except spleen, brain cortex and heart. There was a close correlation between tissue 11 beta-HSD mRNA levels and activity (r = 0.91, P less than 0.001) suggesting pretranslational regulation of the enzyme at a tissue level. There was also close co-localization of GR mRNA (7 kB), measured by hybridization to a rat GR cRNA probe, and enzyme mRNA/activity in every tissue studied except heart and brain cortex in which GR mRNA was found. In the mineralocorticoid target tissues kidney and colon, additional 11 beta-HSD mRNA bands were seen (kidney 1.8 kB, colon 3.4 kB), suggesting the presence of multiple dehydrogenase species. 11 beta-HSD is widely distributed and suitably placed to modulate ligand occupancy of the GR. The possibility of multiple dehydrogenase species in mineralocorticoid target tissues is consistent with the hypothesis that the ubiquitous 'native' 1.4 kB hepatic enzyme regulates the GR, and these separate dehydrogenases regulate the MR.