The binding characteristics of glucocorticoids to cytoplasmic and nuclear fractions of the adenohypophysis were determined and an attempt was made to correlate steroid uptake with physiological action. Binding was of high affinity and exhibited a limited number of acceptor sites for dexamethasone (Dx). It was shown that the steroid-receptor complex dissociated rapidly at 37 °C, but was stable at low temperatures. An inverse relationship was detected between the plasma corticosterone (B) titer and the capacity of receptors to combine 3H-Dx. Shortly after stress or injection of physiological doses of B, 50 to 70% of the acceptor sites were saturated. Progressive desaturation of the occupied receptors appeared to depend upon time and the plasma B concentration. Some evidence was also provided supporting a possible correlation between the extent of Dx inhibition of in vitro ACTH secretion and the degree of binding sites’ occupancy. These results suggest that glucocorticoid binding to pituitary receptors may be correlated with the physiological action of the steroids on ACTH relea