Immunoreactive ACTH (I-ACTH) concentrations in the anterior pituitary (AP), intermediate-posterior pituitary (IP) and plasma, and immunoreactive corticotropin-releasing factor (I-CRF) concentrations in the median eminence (ME) and IP, were determined in adrenalectomized rats from 3 h till 14 days after surgery. Plasma I-ACTH concentrations showed the typical triphasic response over time. AP I-ACTH concentrations decreased immediately after surgery, then increased to high concentrations 3 days after surgery. I-ACTH concentrations in IP did not change through these periods. I-CRF concentrations in ME and IP decreased immediately after surgery, then gradually increased to high concentrations (ME) or to control levels (IP) 14 days after surgery. These results raise the possibility that the I-CRF in IP is of hypothalamic origin.