SUMMARY: The surgical removal of embryos from the uteri of sheep at selected stages from the 5th to the 15th day of pregnancy provided a method of studying the functional relationship between the embryo and the corpus luteum. The cyclical life-span of the corpus luteum was not affected by the presence of embryos in the uterus provided that they were removed by the 12th day after oestrus. However, a delay in the removal of the embryos until day 13, 14 or 15 resulted in a marked extension of luteal function. These findings indicate that the luteotrophic effect exerted by the embryo first becomes essential for the maintenance of the corpus luteum of pregnancy after day 12 and are in close agreement with previous results obtained by the transplantation of embryos.