To determine if changes in endometrial expression of the enzymes and receptors involved in prostaglandin (PG) synthesis and action might provide insights into the PGs involved in the initiation of decidualization, ovariectomized steroid-treated rats at the equivalent of day 5 of pseudopregnancy were given a deciduogenic stimulus and killed at various times up to 32 h thereafter. The expression of PG-endoperoxide synthases (PTGS1 and PTGS2), microsomal PGE synthases (PTGES and PTGES2), cytosolic PGE synthase (PTGES3), prostacyclin synthase (PTGIS), prostacyclin receptor, peroxisome proliferator-activated receptor delta (PPARD) and retinoid x receptor alpha (RXRA) in endometrium was assessed by semiquantitative RT-PCR, western blot analyses and immunohistochemistry. In addition, to determine which PG is involved in mediating decidualization, we compared the ability of PGE(2), stable analogues of PGI(2), L165041 (an agonist of PPARD), and docasahexanoic acid (an agonist of RXRA) to increase endometrial vascular permeability (EVP, an early event in decidualization), and decidualization when infused into the uterine horns of rats sensitized for the decidual cell reaction (DCR). EVP was assessed by uterine concentrations of Evans blue 10 h after initiation of infusions. DCR was assessed by the uterine mass 5 days after the initiation of the infusions. Because enzymes associated with the synthesis of PGE(2), including PTGS2, are up-regulated in response to a deciduogenic stimulus and because PGE(2) was more effective than the PGI(2) analogues and PPARD and RXRA agonists in increasing EVP and inducing decidualization, we suggest that PGE(2) is most likely the PG involved in the initiation of decidualization in the rat.