Mineralocorticoid and glucocorticoid receptors were measured in circulating mononuclear leukocytes in 5 patients affected by Conn’s syndrome (3 cases of bilateral adrenal hyperplasia and 2 cases of adenoma plus unilateral hyperplasia). The number of the binding sites per cell resulted significantly lower (189 ± 114, mean ± SD), as compared with the normal controls (298 ± 105. The affinity of aldosterone for the receptor was found to be not different than that of healthy control subjects. The capacity and the affinity of dexamethasone for glucocorticoid receptors ranged in the normal values. These data suggest a possible down-regulation of mineralocorticoid receptors in humans.