Chorionic gonadotrophins seem to be unique for primate and equid species. Unlike primates, the equine conceptus does not implant in the maternal uterine endometrium until around day 37 of pregnancy. At this time specialized cells of the trophoblast, organized in the embryonic girdle, invade the endometrium and become established in the endometrial stroma, forming the so-called endometrial cups. This migration of girdle cells is accompanied by their morphological transformation into large decidual-like cells and by the appearance of a gonadotrophic hormone in the mare's blood. There is convincing evidence today that the hormone is of chorionic origin; therefore the term equine Chorionic Gonadotrophin (eCG) seems to be more appropriate than the formerly used term Pregnant Mare Serum Gonadotrophin (PMSG). Secretion of eCG peaks between days 60 and 80 in pregnant mares, to decline gradually until day 130 of gestation, with pronounced inter-individual variation. There appear to be no hormonal regulatory mechanisms controlling eCG synthesis and secretion, suggesting that the size and the morphology of the endometrial cups are the limiting factors. Equine CG is a glycoprotein hormone, composed of noncovalently bound alpha- and beta-subunits. The alpha-subunit consists of 96 amino acids and is identical for eCG and the pituitary hormones eLH, eFSH, and eTSH. The beta-subunit is similar to beta-hCG in that both have a C-terminal extension. It is comprised of 149 amino acids and the peptide primary structure is identical to that of beta-eLH.(ABSTRACT TRUNCATED AT 250 WORDS)