Dexamethasone, corticosterone and deoxycorticos-terone were given to intact, encephalectomized or decapitated rat fetuses from mothers treated with metyrapone, in order to deter-mine the location of the sites of the negative feedback action of corticosteroids. The corticosterone concentration dropped in the adrenals and plasma of intact fetuses, measured on the 22nd day of pregnancy (21-day-old fetuses), after metyrapone (30 mg) had been given to the mothers on the 21st day of pregnancy (20-day-old fetuses). Adrenal hypertrophy was observed in intact (35%) and encephalectomized (18%) but not in decapitated fetuses. Corticosterone acetate (100 µg) prevented adrenal hypertrophy in both intact and encephalectomized fetuses. Dexamethasone acetate (125 µg) prevented adrenal hypertrophy only in the intact fetuses. A high level of deoxycorticosterone acetate (600 µg) also prevented adrenal hypertrophy. These results are discussed. These data suggest that the pituitary as well as the hypothalamus and/or the upper nervous structures contain sites sensitive to the negative feedback action of corticosterone. They also point toward a hypothalamic site of action of dexamethasone.