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The stage at which the embryo first affects the corpus luteum of the sheep in such a way as to prevent its regression at the end of the oestrous cycle was determined by means of embryo transplantation. Up to the 12th day after oestrus the transfer of embryos to non-pregnant ewes resulted consistently in pregnancy, an indication that the transferred embryos had a luteotrophic effect on the corpora lutea of the recipients. However, embryos transferred to recipients in which oestrus had occurred 13 or 14 days previously had only a slight, or more frequently, no effect on the life-span of the corpus luteum. The factor responsible for the failure of these transfers was shown to lie in the recipient ewes and not in the transferred embryo. The findings indicate that the presence of the embryo in the uterus between the 12th and 13th day after oestrus is essential for the maintenance of the corpus luteum of pregnancy.