Abstract: The concentrations and cyclic variation of 6-keto-PGF₁ and TXB₂. metabolites of PGI₂ and TXA_z, were determined, using radioimmunoassay, in the endometrium of 86 normal women （156 specimens） and at the same time the PGI₂/TXI₂ ratio was assessed. It was found that the TXA₂ content was lower than PGI₂ through out the cycle except the menstrual period and no cyclic va riation. But the PGI₂ content was significantly higher in the secretory phase than in the proliferative phase and it is also formed two peaks in early and late secretory phase. Just following the peaks of estrogen and progesterone, there was cyclic variation. Both PGI₂ and TXA₂ content reached to their highest levels in menstual period, especislly TXA₂; leading to reversed PGI₂/TXA₂ ratio indieating that TXA₂ may play a role in the control of menstusl blood loss. PGI₂ and TXA₂ contents in different ages were also observed, finding that the TXA₂ level increased with age.The role of these two PGs played to the development of endometrium and their biological significance in hemostatic mechanism during menstrunation were discussed. It was suggected that alteration of the PGI₂/TXA₂ ratio was a factor which can influcence the menstrual blood loss.