A.R.C. Unit of Reproductive Physiology and Biochemistry, Cambridge (Received 12th November 1974) In sheep, the transfer of two- to four-cell eggs from the oviduct of donors to the uterus of recipients results in a poor conception rate, while uterine transfers of Day-3 (eight-cell) eggs (Day 0 = day of oestrus) give a satisfactory conception rate (Averill & Rowson, 1958). Moore & Shelton (1964) found that transfers to the oviduct were more successful than those to the uterus, but Trounson & Moore (1974) found that the site of transfer of Day-3 eggs had no effect upon their subsequent survival. In sheep, most eggs are present in the uterus 66 hr after ovulation (Holst, 1974), whereas in the cow, which ovulates about 12 hr after the end of standing oestrus, Hamilton & Laing (1946) could find no eggs in the uterus before 96 hr after the end of oestrus. Thus, although the