In cytosol preparations renal mineralocorticoid receptors have equivalent affinity for aldosterone and corticosterone; in vivo, such receptors are clearly aldosterone-selective. To explore this difference, we have compared the binding of both steroids in kidney slices, high speed supernatants and true cytosols in vitro, in the presence of RU28362 to exclude steroids from Type II glucocorticoid receptors, and/or in the presence of cortisol 17 beta-COOH to exclude corticosterone from transcortin. In addition, we have explored the effect of tissue preparation and cortisol 17 beta-COOH on the level of activity of the renal enzyme 11 beta-OH steroid dehydrogenase. The results of the present studies support the hypothesis that the predominant aldosterone specific-conferring mechanism for renal Type I receptors in vivo in the enzyme 11 beta-OH steroid dehydrogenase, and underline the potential difficulties in interpreting studies which include multiple potential ligands to multiple potential binding sites.