Phospholipase A2 (PLA2) and cyclooxygenase (COX) are two key enzymes in PG synthesis; the latter has two forms, COX-1 and COX-2. mRNA was extracted from single preimplantation embryos and examined for PLA2, COX-1, and COX-2 gene ex- pression by RT-PCR to investigate whether PLA2 and COX genes are expressed in human preimplantation conceptuses from zygote to blastocyst stage and to compare COX-1 and COX-2 gene expression within the same stage of embryonic development. Expression of PLA2, COX-1, and COX-2 was de- tected in 48, 37, and 45%, respectively, of total embryos exam- ined. COX-1 was expressed in approximately 66% of early hu- man preimplantation embryos from zygote to two-cell stage, whereas COX-2 was expressed in about 58% of later stage em- bryos from eight-cell to blastocyst stage (P < 0.05). Further- more, COX-2 mRNA and protein were localized to trophecto- derm in blastocyst stage embryos. In conclusion, PLA2, COX-1, and COX-2 are expressed during early human embryonic de- velopment and may contribute to the production of PGs such as PGE2 in human embryogenesis. COX-1 and COX-2 are dif- ferentially expressed, with COX-2 being primarily expressed by trophectoderm in late-stage human preimplantation em- bryos, which may promote embryonic differentiation and implantation. (J Clin Endocrinol Metab 87: 2629 -2634, 2002)