Adrenalectomy (ADX) leads to a decrease in the number of CRF-binding sites in the rat anterior pituitary (AP). However, the molecular mechanisms of CRF receptor (CRF-R) regulation are unknown. In the present study, we analyzed the effects of ADX on pituitary CRF-R1 messenger RNA (mRNA) levels in vivo and the direct effects of CRF, arginine vasopressin (AVP), and glucocorticoid, the levels of which are altered by ADX, on CRF-R1 mRNA levels in vitro. The mRNA level was determined by Northern blot analysis using a rat brain CRF-R1 complementary RNA probe. The CRF-R1 level in AP fell to 20% of the sham level 1 day after ADX and returned to the sham level after 14 days. In cultured rat AP cells, treatment with CRF, AVP, and dexamethasone led to significant reductions in CRF-R1 mRNA, with maximal inhibition to 32%, 22%, and 37% of control levels, respectively. The time course of CRF-R1 mRNA reduction varied depending on the drug, with effects detectable as early as 1 h after treatment. These findings indicate that elevated portal CRF and AVP levels may contribute to the decrease in CRF-R1 mRNA soon after ADX. A decrease in mRNA levels, in turn, may lead to a decrease in CRF-R1 protein on corticotrophs.