In recent years, new theory concerned upon the estriol biosynthesis in the pregnant was developed remarkably. Diczfalusy and others demonstrated that DHA from foetal adrenal was the main precurssor of estriol in the pregnant. Their approaches for this problem were tried mainly with in vitro incubation and circulation studies. In this study, the approach was tried with the determination of steroids in cord blood, maternalblood, maternal urine and urine of newborn. Androstanedion, Δ4-androstenedione, DHA, androsterone and etiocholanolone in cord blood were deter minated by new method, which was cosisted of following procedure. 1) 3s Hydrolysis (free steroid extraction, hydrolysis with β-glucuronidase in phosphatete buffer and transesterification with methyl-green and acetic acid. 2) Florisil column chromatography. 3) Thin layer chromatography. 4) Colorimetry following Lieberman, colour development with modified zimmerman reaction. The results of this determination were as following. In two cases, five androgens in cord blood and maternal peripheral blood, eight fractions of estrogens and nine fractions of 17-Ketosteroids in maternal urine, were determined. Also nine fractions of 17-Ketosteroids and four fractions of estrogens were determined in pooled urine of newborn. The results of these determination were as followings. 1) DHA level in cord blood was higher than that in maternal blood. But DHA level in maternal blood was cosiderably high. DHA in cord blood: 88.4μg/100ml, DHA in maternal blood: 37.3μg/100 ml It should be indicated that, thinking of maternal blood volume, maternal DHA is important for estriol biosynthesis. 2) Although DHA level in maternal urine was high, but DHA level in urine of newborn was low. Urinary DHA in mother: 1111.3μg/day, Urinary DHA in newborn: 10.050.0μg/day By this results, it is difficult to think that all estriol should be the product of foetal DHA. 3) Urinary estriol in early post-natal days was relatively high and then decreased to 10 and less μg in seven days of newborn. This indicates that the stored estriol was excreted in post-natal days. By these results, it should be clear that estriol biosynthesis is done not only in foetoplacentar unit, but also in maternalplacentar unit. It is claimed that steroids metabolism in pregnancy, as was seen in estriol biosynthesis, must be examined not as foeto-placentar unit but as foeto-placentar-maternal relation.