Systemically ineffective amounts of a cortisol acetate emulsion or pellets of corticosterone or dexamethasone were placed in various regions of the ventricular system of the rat. One or 2 days later the effect on ether-induced ACTH secretion was estimated indirectly by measuring plasma corticosterone changes. Injections of cortisol acetate or implants of dexamethasone were effective in suppressing ACTH secretion when placed into the lateral ventricle but not if placed immediately adjacent to the ventricle. Injections of cortisol acetate into the temporal horn of the ventricle were effective in suppressing ACTH secretion. Implants of corticosterone acetate in the third ventricle were also effective. Implants of this steroid in the tissue immediately adjacent to the third ventricle were effective, but the effectiveness decreased with increasing distances of the implants laterally from the third ventricle. Injections of cortisol acetate into the cisterna cerebellomedullaris were ineffective. It is suggested ...