In order to test the proposal that the aldosterone specificity of mineralocorticoid receptors in the collecting duct depends on inactivation of glucocorticoids by the enzyme 11beta-hydroxysteroid dehydrogenase (11beta-HSD), we have assessed the effect of pharmacological inhibition of 11beta-HSD on collecting duct Na+ reabsorption in vivo. Adrenalectomized rats (n=14) were infused intravenously with high-dose corticosterone, and late-distal tubules were perfused orthogradely with artificial tubular fluid containing [14C]inulin and 22Na; urinary recoveries of the radioisotopes were monitored. Half of the rats received intravenous carbenoxolone to inhibit renal 11beta-HSD activity. The urinary recovery of [14C]inulin was complete in both groups of animals (101+/-2% versus 101+/-3%), but the recovery of 22Na was lower in carbenoxolone-treated rats (34+/-5%) than in the corticosterone-alone group (54+/-4%, P<0.01). These data, which provide the first demonstration of enhanced Na+ reabsorption in the distal nephron during inhibition of renal 11beta-HSD in vivo, strongly support the proposal that 11beta-HSD normally prevents endogenous glucocorticoid from exerting mineralocorticoid-like effects.