Several previous studies have demonstrated that uterine Cox2 (also known as Ptgs2) is required for implantation. Luteinizing hormone (LH) released from anterior pituitary gland and human chorionic gonadotropin released from placenta (hCG) can upregulate the uterine Cox2 gene expression. The Lhcgr knockout (herein designated LHRKO) animals have implantation failure even after estradiol and progesterone therapy. These findings led us to investigate the dependence of uterine Cox2 gene expression on LH signaling in LHRKO animals. The results revealed that, while Cox1 (also known as Ptgs1) mRNA levels were similar, Cox2 mRNA levels were lower in uterus of null animals than in wild-type siblings. Treatment with hCG did not increase Cox2 mRNA levels in null endometrial stromal or myometrial smooth-muscle cells unless gene therapy was performed to introduce native LHCGR. The Cox1 mRNA levels, on the other hand, did not change regardless of the introduction of native or activated Lhcgr or hCG treatment. The Cox2 mRNA increase paralleled the cAMP raise, suggesting that LH uses the cAMP second messenger system. Treating the wild-type uterine cells with hCG resulted in a Cox2 but not Cox1 mRNA increase. This increase became exaggerated when additional native LHCGR were introduced by gene therapy. In conclusion, deletion and reinsertion of Lhcgr further support that uterine Cox2 gene expression is dependent on LH signaling.