Embryonic signals not only rescue the corpus luteum but also modulate the uterine environment in preparation for implantation. Chorionic gonadotropin (CG), an early embryonic signal, is one such molecule. In vivo studies in the baboon show that CG alters the morphology and biochemical activity of the uterus, especially with respect to the three major cell types: luminal epithelium, glandular epithelium, and stromal fibroblasts. Further, CG and progesterone have a synergistic effect on the receptive endometrium. CG action on the endometrial cells is transduced via a seven transmembrane G protein-coupled receptor. Possible signaling pathways involved are discussed.