The 11 beta-dehydrogenase (11-DH) component of the microsomal enzyme complex, 11 beta-hydroxysteroid dehydrogenase, has been postulated to be the specificity-conferring mechanism for the renal type I receptor. To this end, the distribution of 11-DH and the type I receptor was studied in rat kidney by immunocytochemistry. 11-DH-like immunostaining (LI) was found in Bouin's fixed tissue, in both nuclear and cytoplasmic components of some cells of the juxtamedullary proximal convoluted tubules, and in interstitial cells of the papillary medulla. In addition, weak staining was seen in superficial proximal convoluted tubules in paraformaldehyde-fixed tissue. Type I receptor-LI was found in distal convoluted tubules, connecting pieces, and initial cortical collecting tubules of the superficial cortex. Since colocalization of the enzyme and receptor was not demonstrated, the action of 11-DH on type I receptor specificity would, thus, appear to be paracrine rather than autocrine.