Embryo transfer studies in the farm mammals have clearly shown that the survival of embryos depends on their stage of development corresponding closely with the developmental stage of the recipient’s uterus. However, much remains in determining the precise nature of many factors involved in the survival of a bovine embryo after transfer; it may not always be a question of simply providing exact synchrony of estrus between donor and recipient. In the early days of surgical embryo transfer, recipient cattle were almost invariably selected on the basis of a naturally occurring estrus. More recently, however, selection of recipients has been widened to include cattle that have their estrous cycles controlled by hormonal means. The synchronizing agents employed must clearly not result in an adverse effect on pregnancy rates in such recipients.