There are numerous reports concerning the metabolism of estrogen in the foetus-placenta system such as the report by Diczfalusy et al. among others, but there still remain many problems unsolved as to the role of steroids that have eluded the conversion of DHA to estrogen and as to how the estrogen metabolism is conducted in the mother-placenta system. For the purpose to elucidate these problems the author studied the foetus-placenta system and the motherplacenta system each as a separate entity using labeled DHA. The results of the study are briefly summarized as follows. 1) It has been demonstrated that the conversion of androgen and estrogen from DHA into 11 steroids is being carried out in the mother's side as well as in the foetus side. 2) The activity of 16α-hydroxylase is higher in the foetus side than in the mother's side. 3) Estrogen derived from the mother-placenta system amounts to about 30% in the middle stage of pregnancy. 4) The rate of synthesis from DHA to estrogen is faster in the foetus-placenta system. 5) There can be observed no difference in the rate of androgen synthesis from DHA between the two systems. 6) The conversion of DHA to estriol is higher in the foetus-placenta system (about 2.5 times that in the mother-placenta system). 7) The conversion percentage of 16-epi-estriol, estradiol and estrone is higher in the mother-placenta system. 8) There is no significant difference in the fraction percentage of androgen between the two systems.