Increased prostaglandin (PG) synthesis by fetal membranes occurs at parturition. PGE(2) synthesis from arachidonic acid involves multiple enzymes and two isoforms of the terminal enzyme of this biosynthetic pathway, PGE synthase (PGES), were recently identified. Cytosolic PGES (cPGES) is identical to the heat shock protein 90 chaperone, p23, and is reportedly functionally coupled to constitutive PG endoperoxide H synthase-1. Microsomal PGES (mPGES) is inducible by proinflammatory cytokines such as IL-1 beta. We have studied expression and localization of both enzyme isoforms in human fetal membranes either at term or preterm, with or without labor. The cPGES was immunolocalized in the amnion epithelium, and associated with fibroblasts and macrophages in the choriodecidual layer, whereas mPGES was localized in the amnion epithelium as well as the chorion trophoblast. Both enzymes were found to be associated with lipid particles present in the amnion epithelium, which are more prevalent in term tissues. Western blot analysis of the amnion and choriodecidua showed no differences in amounts of either cPGES or mPGES at term or preterm, with or without labor, in either tissue with advancing gestation. It does not appear that expression of PGES is the rate-limiting step in PGE2 synthesis in fetal membranes at labor.