Plasma levels of corticosterone, obtained prior to and following 15 min sham or electrical stimulation (0.1–0.2mA), were determined fluorometrically for adult female rats which had been pharmacologically blocked with combined dexamethasone (l mg/kg), morphine (20 mg/kg) and pentobarbital (30 mg/kg). Whereas, stimulation of neural sites within the diagonal band of Broca, suprachiasmatic, arcuate, paraventricular and dorsal premammillary nuclei produced significant increases in plasma levels of corticosterone, sham stimulation as well as electrical stimulation of other forebrain areas produced no change in the suppressed levels of plasma corticosterone. These data suggest that the hypothalamic elements immediately involved in the release of ACTH are not distributed diffusely throughout the ventromedial hypothalamus in the usual sense but are localized to discrete hypothalamic sites generally close to the midline. (Endocrinology93: 835, 1973)