Prostaglandins (PGs) are important in blastocyst implantation and decidualization. The level of PGI₂ was the highest followed by PGE₂ in the site of blastocyst implantation. In addition, PGI₂ level was significantly higher at the implantation sites than inter-implantation sites. IP and PPARs are the G protein-coupled cell surface receptors that are linked to different cytoplasmic signaling pathways and the nuclear hormone receptors, respectively. IP is not expressed or undetectable at implantation sites, but PPARδ expression is very abundant. RXRs (the partner receptor for PPARs), PPARδ-RXRα heterodimer and PGI₂ synthase (COX-2/PGIS) are also abundant at implantation sites. Both PGI₂ agonist (cPGI) and PPARδ specific agonist can restore blastocyst implantation and decidualization in COX-2^－/－ mice. These data suggested that PGI₂ may play important roles in blastocyst implantation and decidualization by PPARδ activation.