Small amounts of crystalline cortisol were implanted into the brain stem or the pituitary gland of rabbits; subsequently the adrenal response to immobilization stress was measured (as indicated by the rise in plasma corticoids) at weekly intervals for several weeks after implantation. The stress-induced adrenal activation was inhibited by implants into the anterior portion of the median eminence, the post-optic region and, to a lesser extent, the antero-medial part of the hypothalamus, between the supra-optic and paraventricular nuclei and the median eminence. No observable effect was induced by similar implants into the posterior hypothalamus, mesencephalon and adenohypophysis. Little correlation could be noted between inhibition of adrenal function and decline of adrenal weight.