Response to immobilization stress was studied in intact, hypophysectomized and hemidecorticated (bearing removal of the left cortical hemisphere) male rats. Blood and adrenal corticosterone content was measured fluorimetrically in these animals, killed at different intervals after beginning of stress. In intact animals, a monophasic curve, stabilized after 10 min, was obtained in blood, and a biphasic response occurred in the adrenal gland, with similar peaks at 5 and 30 min. In hypophysectomized animals a decrease of adrenal corticosterone, parallel to a blood increase, was found during the first 10 min. After 15 min the elevation was simultaneous in blood and adrenal glands. In hemidecorticated animals no difference was found between the homo and heterolateral glands to the ablation, but adrenal basal level was higher than that in intact animals. GLC study permitted observation of changes in relative concentrations of different corticosteroids in the above mentioned groups.