The pituitary-adrenocortical system of 21- to 22-day-old rat fetuses was investigated. 1. Adrenal ascorbic acid depletion caused by stressful stimulus was the same in the fetuses of hypophysectomized rats as in those of normal rats. The same degree of depletion (15% in 1 hr) also occurred in fetuses of hypophysectomized rats maintained ex utero at room temperature during the 1-hr period. 2. Fetal adrenals incubated in Krebs- Ringer bicarbonate buffer produced corticosterone at the same rate as did adrenals from adult rats. However, the fetal adrenals were found to be more sensitive to ACTH in vitro than the adrenals of adult rats. 3. Fetal pituitaries increased in vitro production of corticosterone by the adrenals of adult rats. The addition of pitressin to the medium (1 mU/ml) had no effect on the production of corticosterone by adrenal tissue incubated with or without fetal hypophyses. These data show that at term the adrenals of fetuses are stimulated by fetal pituitary. In addition, a definite functional capacity of fetal adrenals and hypophysis in vitro was shown.