Concentrations of FSH, LH, chorionic gonadotrophin (CG) and progesterone were measured during early pregnancy in mares and donkeys carrying normal intraspecies and transferred extraspecies conceptuses. A secondary rise in progesterone concentrations occurred in normal intraspecies horse and donkey pregnancy soon after the appearance of CG but FSH concentrations continued to fluctuate and were not influenced by CG production. In donkeys carrying transferred horse conceptuses FSH concentrations fell sharply and progesterone concentrations rose steeply, coincidental with the appearance of abnormally high levels of CG. In horses carrying donkey conceptuses, on the other hand, there was a complete absence of CG production and in 7 of the 8 mares studied there was no secondary progesterone rise; FSH levels fluctuated as in the intraspecies pregnancies. Pregnancy continued to term in only 1 of 8 mares carrying a donkey conceptus despite attempts to maintain it in 4 animals by exogenous progestagen therapy. Our findings indicate that pituitary FSH, not CG, stimulates secondary follicular growth during normal equine pregnancy and the LH-like activity of CG induces ovulation of these follicles to form secondary corpora lutea.