In a longitudinal study, the changes in plasma concentrations of dehydroepiandrosterone sulfate (DHAS) and total estriol (E3) were followed in 10 normal pregnant women throughout gestation at 8-10, 14-17, 21-24, 28-32, and 38 weeks as well as at the time of admission to the delivery room. The mean plasma concentration of DHAS decreased from 325 +/- 38.2 micrograms/dl in early gestation to minimum levels of 120 +/- 15.9 micrograms/dl at week 38. At delivery there was a 2-fold increase in plasma DHAS (205.3 +/- 17.7 micrograms/dl). The mean total E3 plasma concentration increased from 6.53 +/- 2.5 ng/ml at week 8-10 to 198.6 +/- 30.3 ng/ml prior to delivery. The decrease in the DHAS plasma levels is caused by the elevated metabolic clearance rate throughout pregnancy. The increasing E3 plasma levels are caused by increasing steroid production by the fetoplacental unit and reflect fetal well-being. The peak levels of DHAS and E3 as well as of other steroids of adrenal or placental origin at admission to the delivery room reflect increased maternal and fetal stress with the onset of labor. These longitudinal reference data determined by modern radioimmunoassay methodology are helpful for the prenatal diagnosis of congenital disorders with insufficient or absent fetoplacental function, and in the control of suppression of the fetal pituitary-adrenal axis during dexamethasone treatment in pregnancies with a female fetus affected with congenital adrenal hyperplasia.