The initiation of the fetal adrenocorticotropic activity was investigated in pregnant rats bearing an ACTH—secreting tumor and in pregnant adrenalectomized females. High maternal plasma corticosterone concentrations were paralleled by markedly lighter fetal adrenal weights between the 18th and 22nd day of intrauterine development, but not on the 17th day. The adrenal glands of fetuses from adrenalectomized females were heavier between the 18th and 22nd day of gestation, but not on the 17th day. The results were interpreted as evidence indicating that fetal adrenocorticotropic activity begins between the 17th and 18th day of intrauterine development. (Endocrinology92: 380, 1973)