In 9 women with a normal pregnancy, delivery and puerperium, measurement was made of free estrone (E_1), free estradiol (E_2), free estriol (E_3), free progesterone (P) and total corticosteroids (CDS), continuously in each period. The maternal blood E_1,E_2,E_3,P and CDS levels at 39 weeks' gestation were higher and the concentration of three fractions of estrogen was higher in the order of E_2>E_3>E_1. When a comparison was made of maternal blood levels of various hormones at 39 weeks' gestation and at the onset of labor, a significant increase in E_1 and CDS was observed at the onset of labor. As for maternal blood levels of various hormones with the advance of delivery, an increase in E_1 and CDS was observed at expulsion of the fetus, and the concentration of three fractions of maternal blood estrogen at this time was higher, in the order of E_1>E_2>E_3. Maternal blood levels of various hormones after the expulsion of the placenta decreased rapidly. Particularly, E_2 and E_3 showed a marked decrease, thus suggesting that the fetoplacenta is closely concerned with the production of maternal blood E_2 and E_3.