To demonstrate the presence of mineralocorticoid receptors, specifically aldosterone receptors, iris-ciliary body cytosol from New Zealand albino rabbits was incubated with increasing amounts (0.5–20.0 nM) of radiolabeled aldosterone in the presence and absence of a 100-fold excess of aldosterone. A glucocorticoid receptor blocker, RU-28362, was employed when various steroids were allowed to compete with aldosterone for binding sites on the aldosterone receptor. Aldosterone bound to two classes of receptor proteins. Scatchard analysis of the data indicated one component to bind aldosterone with high affinity (Kd = 2.2 × 10––9M) and low capacity (Bmax = 32 fmol/mg protein). If the concentration of aldosterone is increased, it also binds to a second component, but with a lower affinity (Kd = 9.8 × 10––9M) and greater capacity (Bmax = 64 fmol/mg protein).