Abstract We have studied the localization of specific cytoplasmic mineralocorticoid receptors in the pituitary and in central structures of the adrenalectomized rat by the use of a specific synthetic glucocorticoid (RU 26988), which does not bind to CBG nor to the mineralocorticoid receptor and which masks type II sites. An in vitro exchange assay with a saturating concentration of [ 3H]-aldosterone, in the presence or absence of a 100-fold excess of radioinert aldosterone and in the presence of a 100-fold excess of RU 26988. revealed that cytosolic mineralocorticoid (type I) receptors follow the pattern: hippocampus > pituitary > septum > amygdala > cortex > hypothalamus > preoptic area = 0. Scatchard analysis of [ 3H]-aldosterone binding in the presence of RU 26988 showed that the mineralocorticoid receptors present in these different structures have approximately the same affinity ( K D = 3.10 −9 M) for aldosterone. The biological significance of this binding, which can be high in certain structures (e.g. hippocampus) is as yet unknown because, as in previous receptor studies, it is difficult to distinguish between the specific mineralocorticoid and glucocorticoid effects of aldosterone. These central mineralocorticoid receptors could mediate salt-appetite induction (hypothalamus), cerebral hypertension and electrolyte regulation in brain and play an important role in cerebral oedema.