The endocrine dialogue that results in implantation and the successful establishment of pregnancy in primates relies on embryonic secretion of chorionic gonadotrophin (CG). This hormone is a signal of embryo viability and capacity to support the corpus luteum. The expression of CG is apparently restricted to primates. Active or passive immunization of marmoset monkeys against the beta subunit of CG prevented implantation and early pregnancy, without disrupting the ovarian cycle. Studies of individual embryos cultured in vitro showed that CG is secreted at low levels by the blastocyst from before attachment, with secretion increasing exponentially after attachment. Gonadotrophin releasing hormone (GnRH) was also secreted, from mid-blastocyst stages, before the detection of CG. The secretion of GnRH by the embryo continued through the attachment and outgrowth stages of embryonic differentiation in vitro. The hypothetical role of GnRH in regulating CG release during implantation was tested in recently completed experiments. Individual embryos cultured with GnRH, or with agonist or antagonist to GnRH, showed significant variations in their secretion of CG and in their survival in culture, suggesting a causal relationship between these hormones. Embryos cultured with natural GnRH showed enhanced growth and development. Embryonic stem cells, from the inner cell mass of marmoset and rhesus monkeys, were the first primate embryonic stem cells to be isolated and characterized, enabling the subsequent isolation of human embryonic stem cells.