Blastocyst implantation in the baboon usually occurs between 8 and 10 days post ovulation. Changes that occur within this window of receptivity and immediately following implantation can be divided into three distinct phases. The first phase, regulated by estrogen and progesterone, is characterized primarily by changes in both the luminal and glandular epithelial cells in preparation for blastocyst apposition and attachment. The second phase is the further modulation of these steroid induced changes in both epithelial and stromal cells by embryonic signals. The final phase is associated with trophoblast invasion and the remodeling of the endometrial stromal compartment. During the initial phase, the actions of estrogen and progesterone are dependent on the presence of specific receptors. Estrogen up-regulates both its own receptor (ER) and the progesterone receptor (PR), while progesterone down-regulates this expression pattern. However, the pattern of progesterone-induced down-regulation of ER and PR is confined to the epithelial cells and demonstrates a gradient effect from the functionalis to the basalis. What is most intriguing is that the loss of epithelial PR is closely correlated with the establishment of uterine receptivity. Coincident with the changes in ER and PR expression, epithelial cells undergo alterations in their cytoskeletal architecture and secretory profile. These changes can be counteracted by PR antagonist treatment during the luteal phase. Although estrogen and progesterone play a critical role in establishing the initial phase of uterine receptivity, it is becoming increasingly evident that the embryo induces functional receptivity in ruminants and rodents. In our studies in the primate, we demonstrate that chorionic gonadotrophin when infused in a manner that mimics blastocyst transit, has physiological effects on the three major cell types in the uterine endometrium. The luminal epithelium undergoes endoreplication and distinct epithelial plaques are evident. The glandular epithelium responds by inducing transcriptional and post-translational modifications in the major secretory product, glycodelin. The stromal fibroblasts initiate their differentiation process into a decidual phenotype and are characterized by the expression of actin filaments. In phase three, blastocyst attachment to the surface epithelium and subsequent implantation is associated with local remodeling of the maternal stroma, smooth muscle, and endothelium of the blood vessels by the trophoblast. In addition, there is a gradual diminution of the epithelial plaques on the luminal surface although the glandular epithelium remains highly secretory. The most dramatic effect is on the stromal fibroblasts, which in response to embryonic stimuli, differentiate into decidual cells, the major cell type of the gestational endometrium. This differentiation is characterized by the expression of insulin-like growth factor binding protein-1 (IGFBP-1) in these cells. The cytokine IL-1 beta is one possible embryonic signal. COX-2 is the rate-limiting enzyme for prostaglandin biosynthesis and transcription of this enzyme in response to the embryonic stimulus (IL-1 beta) results in an increase in prostaglandin biosynthesis in stromal fibroblasts at the site of implantation. Prostaglandins and PGE2 in particular, binds to its specific receptor (EP2 or EP4) and activates adenyl cyclase. The resulting increase in intracellular levels of cAMP can now activate IGFBP-1 gene transcription at the site of implantation. In summary, our studies have demonstrated that chorionic gonadotrophin, when infused into non-pregnant baboons during the window of uterine receptivity can induce epithelial responses that are similar to those observed in a fertile cycle. Stromal differentiation is initiated; however, decidualization requires a signal from the conceptus.