The relative role of maternal and fetal adrenal function in the production of maternal urinary estriol during pregnancy was studied by transuterine, intramuscular injection of adrenocorticotropin (ACTH) into the fetus and intravenous administration of ACTH to the mother. Normal untreated patients and patients in whom adrenal function was suppressed by administering dexamethasone to the mother served as subjects. At no time was urinary estriol excretion increased. It is suggested that an increase did not occur in normal patients because the enzyme involved in estriol synthesis was operating near maximum capacity. In patients who had taken dexamethasone prior to injection of ACTH, the quantity of ACTH might not have been sufficient to overcome the suppressive action of dexamethasone.