The concentration of corticotropin-releasing factor (CRF)-binding sites decreases in the rat anterior pituitary after adrenalectomy; this change may be related either to a direct effect of the circulating glucocorticoids at the pituitary level or to a desensitization of CRF receptors through an increased CRF release in hypophysial portal blood. In order to examine the latter possibility we have measured plasma adrenocorticotropin hormone (ACTH) levels and the number of anterior pituitary CRF binding sites in sham-operated and 24-hour adrenalectomized rats after blockade of endogenous CRF by passive immunization with an antiserum anti-rat CRF (CRF-AS), or after injection of normal rabbit serum (NRS). In NRS-injected rats, after sham operation, plasma ACTH concentration increased (227 + 34 vs. 118 ± 19 pg/ml in controls) without change in CRF-binding sites capacity (20.7 ± 2.6 vs. 24.6 ± 3.5 fmol/mg protein in controls). Adrenalectomy induced a large rise in plasma ACTH (785 ± 89 pg/ml) and a decrease in the number of CRF-binding sites (12.2 ± 1.7 fmol/mg protein). After CRF-AS injection, plasma ACTH was normalized in sham-operated animals (149 ± 24 pg/ml) and significantly reduced in adrenalectomized rats (472 ± 76 pg/ml); the adrenalectomy-induced decrease in the number of CRF-binding sites was unaffected by the CRF-AS administration (12.2 ± 1.7 fmol/mg protein). The administration of dexamethasone to adrenalectomized rats significantly reduced plasma ACTH concentrations (23.3 ± 10.6 pg/ml) and prevented the loss in CRF-binding sites capacity (20.7 + 1.3 fmol/mg protein). Affinity of CRF for its binding sites was unchanged in all the groups studied. These findings suggest that the postadrenalectomy reduction in the number of anterior pituitary CRF-binding sites may be mainly due to a direct glucocorticoid effect at the pituitary level rather than a consequence of a possible increased CRF s