We determined the corticotropin-releasing factor(CRF)-like immunoreactivity by radioimmunoassay in intermediate and posterior pituitary lobes of stalk-sectioned and sham-operated control rats. The antigenic determinant read by the CRF antibody used was contained within the region of amino acids 26 (Gln) to 37 (Lcu) ofthe molecule. Intermediate and posterior lobes of control rats contained similar amounts of CRF (591 + 78 and 487 ! 34 pg/mg protein, respectively). The section ofthe pituitary stalk p¡oduced a marked dec¡ease in CRFlike immunoreactivity in both structures. The CRF content on the intermediate lobe after stalk section was 62 t l7 pg/ng ptotein, a decrease of 90%, and that of the posterior lobe was 90 + 13 pg,hg protein, an 83% decrease. Our results suggest that most ofthe CRF-like immunoreactivity in the ¡at intermediate and posterior pituitary lobes is contained in nerve fibers ofbrain origin and support the hypothesis of a ¡ole of CRF in the release of intermediate and posterior lobe peptides.