1. The use of the receptor stabilizing agent sodium molybdate, and of RU26988 to exclude [3H]-aldosterone binding from Type II glucocorticoid receptors, has enabled the characterization of high affinity Type I aldosterone binding sites in rat atrial and ventricular cytosols. 2. In adult male and female rats the affinity of binding (Kd 4 degrees C) is approximately 1-2 nmol/l for both atria and ventricles; specificity of binding is similar to that for Type I sites in classical aldosterone target tissues (aldosterone = corticosterone much greater than dexamethasone). 3. Levels of atrial Type I sites are higher than the corresponding levels in ventricle in both males and females, whereas for Type II (classical glucocorticoid) receptors the reverse is the case; levels of both Type I and Type II sites fall over the age range examined (40 days-6 months). 4. The physiological function(s) of cardiac Type I sites, and their in vivo mineralocorticoid or glucocorticoid selectivity, remain to be explored.