Facilitatory and inhibitory components of the nervous system have been postulated for the regulation of ACTH. Some of these have been activated in the present study by electrical stimulation in two brain stem areas in cats prepared as encéphale isolés. 30 second stimulation in the ventral midbrain teg-mentum evoked a specific and rapid decrease in adrenal effluent corticosteroids of 7 cats; stimulation in the posterior diencephalon in 17 cats was followed by an increase, a decrease or no significant change in corticosteroid levels. An interrelationship between midbrain and diencephalic areas concerned with the regulation of ACTH release appears evident on the basis of reversals in corti-costeroid levels in response to combined stimulation. That the inhibitory and facilitatory effects on corticosteroid release are neurological in origin rather than due to alteration of peripheral mechanisms seems evident on the basis of the rapidity of response and the lack of correlation with blood pressure changes.