The simultaneous determination of estradiol, estrone, progesterone, and prostaglandins E and F has been made in fatal and maternal compartments in patients with and without the onset of labor. The decidua rather than the fetus or placenta was considered to be the site of prostaglandin synthesis, and prostaglandins present in the amniotic fluid during labor were thought to be a byproduct of myometrial activity rather than the factor initiating the onset of labor. Progesterone levels in the maternal plasma were lower during labor but estradiol levels were elevated. It was concluded that the steroid environment may contribute to the clinical course of labor by facilitating the local uterine production of prostaglandins. Whereas estradiol was high and estrone low in maternal circulation, the ratio was reversed in the fetus. This reversal may serve to protect the fetus from high maternal levels of estradiol.