The influence of decapitation or encephalectomy (total removal of the brain leaving the pituitary in place) on the adrenal corticosterone concentration of the 20-day-old rat fetus has been studied in normal pregnant rats, in adrenalectomized pregnant rats, and in adrenalectomized pregnant rats subjected to the stress conditions of inhalation of ether for 40 min. Decapitation or encephalectomy of the fetus always results in a drop in adrenal corticosterone concentration within 4 h which is prevented in 15 min by injecting 3.2 mU of hog ACTH into the decapitated fetus. In mothers adrenalectomized in order to avoid a negative feedback reaction of maternal corticosteroids on the fetal pituitary-adrenal system, ether inhalation causes an important rise in adrenal corticosterone concentration in normal fetuses but not in decapitated or encephalectomized ones. Thus ether, which crosses the placental barrier, is a stressor agent for the fetal rat.