Abstract The ontogeny of [ 3Hcorticosterone and [ 3Hdexamethasone receptor macromolecules in cytosol derived from brains of adrenalectomized rats 2–30 days of age was studied. The binding of [ 3Hcorticosterone in male rats perfused at sacrifice with ice-cold isotonic sucrose in buffer, 2–8 days of age, ranged from 100 to 133 fmoles/mg protein. Binding increased from 133 to 356 fmoles/mg protein between 8 and 19 days of age, then decreased slightly between 19 and 30 days of age. [ 3HDexamethasone binding under similar conditions was comparable to that of corticosterone from 3 to 8 days of age, but in older animals the amount of receptor-bound [ 3Hdexamethasone was two-thirds that of corticosterone. Binding of these steroids in female rats was similar to that in male rats. The apparent dissociation constants for [ 3Hcorticosterone and [ 3Hdexamethasone were equal, and they did not change with age. In all age groups studied, unlabeled corticosterone competed more strongly for [ 3Hcorticosterone binding sites than did unlabeled dexamethasone, but both steroids competed equally well for [ 3Hdexamethasone binding sites. Although qualitatively similar at all ages, competition by unlabeled corticosterone and dexamethasone binding sites showed quantitative differences as the animal matured. For example, there was less competition for binding sites in 5-day-old animals than in older animals. These results indicate that binding sites for [ 3Hcorticosterone and [ 3Hdexamethasone in neonatal brain may be less specific than in the more mature brain. Also, the increase in the number of binding sites with age can be correlated with known physiological changes in the developing pituitary-adrenal axis.