THE binding of a drug with its specific macromolecular receptor(s) seems to constitute the first step whereby a number of pharmacological agents initiate a variety of intra and intercellular processes. Aldosterone is the principal mineralocorticoid in mammals regulating ion transport in the kidney, and some extrarenal targets, but at higher concentrations it also exhibits glucocorticoid-specific properties1. From studies on displaceable binding to protein ligands alone, it is not possible to affirm the physicochemical nature and homogeneity of a ‘specific’ mineralocorticoid receptor that is thought to be essential for the steroid function and said to exist in the toad bladder2 and rat kidney3. Techniques have been established for the characterisation and partial purification of corticosterone receptors in rat liver4,5. These have now been adapted to demonstrate aldosterone-specific binding proteins present in rat kidney but absent from the liver.