Equine chorionic gonadotropin (eCG) was discovered more than 80 y ago as a factor found in circulation of the pregnant mare during the first third of gestation. It is a variant of equine luteinizing hormone (LH), differentially glycosylated by the equine trophoblast cells. It has the peculiar property of provoking both follicle-stimulating hormone (FSH) and LH activity in non-equid species. The biological basis for this dual activity is believed to be the result of promiscuity of the mammalian FSH receptors, imparting the capacity to respond to this equine LH-like hormone. The best approximation of the role of eCG in the mare is that it induces accessory corpora lutea to better support early gestation. There are numerous applications for eCG in domestic species, including induction of puberty, reversal of anestrus, superovulation and, most recently, improvement of fertility