The inhibitory effects of corticosterone, dexamethasone and prednisolone on activity of the hypothalamus-pituitary-adrenal axis were investigated in adrenalectomized rats infused with glucocorticoids for 6h. Infusion of 202 μg corticosterone did not inhibit the plasma ACTH concentration, but 504 μg corticosterone significantly suppressed plasma ACTH levels. Infusion of 20 μg dexamethasone suppressed markedly the plasma ACTH concentration. These data suggest that the degree of inhibition of dexamethasone on ACTH release is about 25 times greater than that of corticosterone. The CRF content of the hypothalamus was not decreased by the administration of 202 μg corticosterone over a 6-hour period, but it was significantly diminished by 504 μg corticosterone. Infusion of 504 μg dexamethasone did not decrease the hypothalamic CRF content; however, infusion of 5 mg dexamethasone effectively suppressed the hypothalamic CRF content. Infusion of 2.5 mg prednisolone did not either decrease the CRF content. These data suggest that the degree of inhibition of natural steroid at the hypothalamus level is stronger than that of synthetic steroids. In rats pretreated with a single injection of dexamethasone (25 μg/200 g boby weight) 22 h prior to the experiments, continuous infusion of 318 μg of dexamethasone significantly suppressed the hypothalamic CRF content, whereas infusion of 504 μg of dexamethasone failed to decrease the hypothalamic CRF content in the rats not pretreated with dexamethasone. This finding suggests that a latent period after the injection of dexamethasone is needed for the appearance of the inhibitory action of synthetic steriods at the level of hypo