The medial-dorsal posterior hypothalamus contains two areas in which electrical stimulation at 200 μamp for 20 sec led to significant increases in peripheral venous concentrations of ACTH 1.5 min after stimulation and two areas in which electrical stimulation led to significant decreases (p < 0.002 in all cases). When either the midbrain just posterior to the responding region or the hypothalamus 1.5 mm lateral to it was stimulated using identical parameters, the peripheral concentration of ACTH did not change 1.5 min after stimulation. Previous studies have shown that the activity of neurons in medial-dorsal posterior hypothalamus is modified by inputs from receptors in the right atrium and in the carotid arteries, two receptor areas implicated in the release of ACTH in response to hemorrhage. These findings suggest 1) that the medial-dorsal posterior hypothalamus has a well-defined, though complicated, organization with respect to the control of ACTH, and 2) that both inhibitory and facilitatory systems are involved in controlling the release of ACTH. In light of other electrophysiological evidence, this area may be important in modulating the release of ACTH in response to changes in blood volume. © 1974 by The Endocrine Society.