To examine the role and interrelations of areas of the medial hypothalamus in the control of release of ACTH, 695 sites in the hypothalamus of 91 cats anesthetized with chloralose-urethane were stimulated electrically (20 s train 200 .mu.A amplitude at 100 Hz). Changes in ACTH were measured by radioimmunoassay. Responses of arterial pressure could not account for changes in ACTH release. Several ACTH-active areas were defined. The anatomical relations of these areas with known nuclei and pathways then were considered. Two ACTH facilitatory areas and one ACTH inhibitory area were identified in the lateral aspect of the medial hypothalamus. The dorsal facilitatory area appears to be an extension of the lateral division of the dorsolongitudinal fasciculus and to extend medially to join the Fields of Forel, the ventral tegmental area of Tsai, and the parvocellular, paraventricular, and periventricular nuclei. The ACTH inhibitory area appears to be an extension of portions of the central tegmental tract and to extend medially to the posterior hypothalamic area and the dorsal hypothalamic area and ventrally toward the basal hypothalamus. The ventral ACTH facilitatory area appears to be coincident with the medial forebrain bundle and to extend anteroventrally and medially through the supraoptic decussation to the suprachiasmatic, ventromedial, dorsomedial, periventricular, infundibular and premammillary nuclei. Stimulation of the median eminence led to increased release of ACTH. Ascending pathways from the lower brainstem mediating control of ACTH may project to discrete areas of the hypothalamus and then converge on the medial basal hypothalamus.