The plasma membranes of several mammalian tissues including the brain are known to have specific binding sites for glucocorticoids. The developmental changes in specific glucocorticoid binding to synaptic plasma membrane (SPM) from rat brain were determined at various postnatal ages, using [3H]triamcinolone acetonide (TA) as the steroid ligand. The specific binding of the labeled glucocorticoid to SPM during the first 2 postnatal weeks was only 40% of the adult level. An increase of the specific binding occurred after day 15, and this developmental rise of binding reached the adult level approximately by the end of the fourth week. Methodologically, these developmental data are detailed in the present article to include nonspecific binding as well as specific binding. Scatchard analysis indicates that the developmental rise of the specific glucocorticoid binding was due to an increase in the membrane binding sites. The ontogenetic increase of membrane binding sites during postnatal brain development provides additional evidence that these binding sites have physiological significance in brain function.