The clinical importance of maternal urinary estriol excretion has been amply demonstrated in the past. The source of the vastly increased amount of estriol in pregnancy still remains in doubt. The fetal adrenal gland appears to be a major source of estrogen precursors. A woman pregnant with an anencephalic fetus which has an absent or atrophic fetal adrenal cortex received tritiated dehydroepiandrosterone sulfate during a control period and during adrenal suppression. In contrast to the normal pregnancy, the maternal adrenal gland continues to provide estrogen precursors near term.