During the peri-implantation period of pregnancy in primates, chorionic gonadotrophin (CG) is the first clear signal of the embryo's presence and viability. In the marmoset (Callithrix jacchus) implantation begins on Day 11-12 after ovulation and CG is secreted by the embryo from this time. The inner cell mass is necessary for the normal secretion of CG by the trophoblast. Implantation can be disrupted both in vivo and in vitro by antisera to the hCG-beta subunit. The secretion of a platelet activating factor by the preimplantation embryo has yet to be confirmed, as has the physiological function of this and other preimplantation signals. Local sampling by perfusion of the corpus luteum allows a direct measurement of the interactions between luteotrophins and luteolysins, as well as a method of screening potential new agents for regulating the function of the corpus luteum.