The central nervous system contains two classes of corticoid receptors, named types I and II following terminology accepted for the kidney. Phenotypically, type I sites are differentiated into a corticosterone (CORT)-preferring species (Ia) and a mineralocorticoid receptor (Ib). These populations were tentatively compared in the spinal cord and hippocampus. Using [3H]dexamethasone (DEX) and selective blockage of sites, we have observed that type II receptors were comparable in both tissues, while Ia was almost exclusive of the hippocampus. Saturation analysis using [3H]DEX demonstrated that type Ia was a low affinity receptor (Kd approximately equal to 2-5 nM) while type II was a higher affinity site (KdII less than KdI). Using [3H]CORT, or [3H]aldosterone (ALDO), as ligand, preferential labeling of type I sites was achieved, always showing higher concentrations in the hippocampus. Therefore, [3H]DEX seems a ligand of choice to visualize types Ia and II receptors. Another difference noted between the spinal cord and hippocampus, pertained to the sensitivity towards the enzyme RNAse A, which increases heat-induced transformation of the bound receptor, according to the results of DNA cellulose affinity chromatography. In these experiments, type I sites of both spinal cord and hippocampus, plus type II of hippocampus, showed sensitivity toward the enzyme, whereas type II of the spinal cord was refractory to RNAse A enhancement of transformation. These results indicate that the dynamics of transformation is different among receptors showing similar affinity and competition, suggesting further heterogeneity due to receptors themselves, or to tissue factors regulating their biochemical properties.