Summary. Conceptuses produce steroids, prostaglandins, proteins and possibly other unidentified agents which may play a role in the establishment and maintenance of pregnancy. A key event in this process is protection of the corpus luteum (CL) from the luteolytic activity of prostaglandin (PG) F-2α of uterine origin. Oestrogens produced by the pig conceptuses between Days 11 and 16 appear to exert an antiluteolytic effect resulting in the sequestering of PGF-2α within the uterine lumen. Failure of the pregnant uterus to release PGF-2α in an endocrine fashion, therefore, allows for maintenance of CL function. Conceptuses of sheep and cattle produce proteins which, when introduced into the uterine lumen of nonpregnant ewes and cows, suppress the ability of oestradiol and oxytocin to stimulate uterine production of PGF-2α. These conceptus secretory proteins appear to exert an antiluteolytic effect by inhibiting uterine production of luteolytic amounts of PGF-2α. The horse conceptus produces both oestrogens and proteins during early pregnancy when uterine production of PGF-2α is suppressed. Co-culture of horse endometrium and conceptus inhibits endometrial production of PGF-2α. Conceptuses of pigs, sheep and cattle undergo elongation to achieve apposition between trophectoderm and endometrium but the horse embryo migrates rapidly and consistently throughout the uterus to achieve endometrial contact.