11 beta-hydroxysteroid dehydrogenase 2 (11 beta-HSD2) protects mineralocorticoid receptors from high circulating levels of glucocorticoids and cDNAs for 11 beta-HSD2 from human and sheep have been recently cloned. We have isolated the gene for mouse 11 beta-HSD2, which spans 7 kilobases and is composed of 5 exons. In comparison to the widely expressed 11 beta-HSD1, its expression is highly restricted and was detected in RNA from adult kidney, colon and small intestine only, and in contrast to sheep was not expressed in the adrenal gland. On in situ hybridization, expression of 11 beta-HSD2 was localized to the distal convoluted tubules and collecting ducts of the kidney, the same compartments of high expression of the mineralocorticoid receptor. 11 beta-HSD2 therefore appears to act in a protective role to exclude glucocorticoids from mineralocorticoid receptors in the kidney.