The biosynthetic study utilizing 14C-eicosatrienoic acid or 14C-arachidonic acid as substrate was carried out with human fetal membranes and villi at term pregnancy. Prostaglandin synthetase activity was demonstrated in the homogenates of decidua, chorionic membrane, amniotic membrane and villi. The enzyme was localized in the microsomal fractions of decidua and amniotic membrane, of which relative activities were 5.1% and 3.1% to that of bovine seminal vesicle microsome, respectively. To achieve the maximal enzyme activity, GSH, Hb and 1-epinephrine must be present. The only type of prostaglandins found in our assay was only Ecompounds, which seemed to be due mainly to the cofactors employed. Production of PGE2 in the microsomal fraction of either decidua or bovine seminal vesicle was inhibited by the 80, 000 × g supernatant fraction of decidua, which was shown to contain an inhibitory substance of heat-unstable and non-dialyzable nature. The significance of the experimental results in assessing the control of PG production in human decidua is discussed.