RNA blot hybridization analysis with cloned rat CRF precursor (prepro-CRF) cDNA as a probe showed that prepro-CRF mRNA existed in rat hypothalamic and extrahypothalamic brain tissue, whereas it was undetectable in the pituitary and adrenal. To study the effect of glucocorticoid on the level of prepro-CRF mRNA in the hypothalmus and that of ACTH/β-lipotropin (βSLPH) precursor mRNA in the pituitary, effects of adrenalectomy and dexamethasone administration were studied in rats. Adrenalectomy markedly raised mRNA coding for ACTH/β3LPH precursor in the anterior pituitary, but not in the neurointermediate pituitary lobe. Hypothalamic prepro- CRF mRNA increased only to 152% of the control value, 7 days after adrenalectomy. The administration of dexamethasone (200 μg/day for 7 days) started immediately after adrenalectomy lowered the ACTH/βSLPH precursor mRNA level in the anterior pituitary to 19% of the intact control value, whereas the level of prepro-CRF mRNA in the hypothalamus decreased only to 102%. These results suggest that glucocorticoids exert their feedback effect at the level of gene expression on both hypothalamic CRF neurons and pituitary corticotropes. Although the possibility that CRF neurons insensitive to glucocorticoid in the hypothalamus might blunt the change in the prepro-CRF mRNA could not be ruled out, it is also possible that the effect of glucocorticoids on the pituitary is dominant. (Endocrinology117: 1314–1320, 1985)