Evidence has been obtained for the existence, under certain conditions, of an ACTH-releasing neurohumor in the peripheral circulation of rats. A technique has been developed for its detection in as little as 4 ml of peripheral blood by the ability of this substance to elevate the blood ACTH of the intact rat. This ACTH-releasing neurohumor was demonstrated in the peripheral blood of hypophysectomized rats but not in blood taken from normal rats. The adrenals were not essential for the elaboration of this factor; however, this neurohumoral agent was influenced by the level of circulating corticosteroids, since it could not be detected in the presence of a high steroid level. The site of origin of this substance was investigated by the use of stereotaxic lesion techniques. The ACTHreleasing neurohumor that was demonstrable in the peripheral blood of hypophysectomized rats could not be detected in these animals following placement of a lesion in the hypothalamus. It is concluded that the ACTHreleasing substance observed in the peripheral circulation of hypophysectomized animals is indeed a specific hypothalamic neurohumor for the regulation of ACTH secretion from the anterior pituitary.