Electrical stimulation of the neural lobe (NL) of the pituitary induces a rise in plasma corticosterone indicating the release of adrenocorticotropin (ACTH) in rats pretreated with dexamethasone, morphine and pentobarbitone. 7-8 days after placing an anterolateral cut around the medial basal hypothalamus (MBH), the rats failed to respond with ACTH release to electrical stimulation of the NL; the number of nerve fibers and terminals in the NL decreased to less than 5% of the normal; and ACTH releasing activity of acid extracts of the NL was undetectable using both in vivo and in vitro tests, which are insensitive to vasopressin. After lesions of the paraventricular nuclei the stimulation of the NL elecited a rise of plasma corticosterone that was significantly less than that in the controls. These results suggest that the NL of the pituitary contains electrically excitable fibers capable of releasing corticotropin-releasing factor (CRF; distinct from vasopressin) and that these fibers probably originate from outside the MBH, with a portion of them coming from or through the paraventricular nuclei or their immediate vincinity.