Varying numbers (2–12) of fertilized eggs from donor mice 3½ days post coitum were transferred to the uteri of pseudopregnant recipients 2½ days post coitum. A higher proportion of blastocysts than of morulae implanted and developed into live foetuses. The number of eggs transferred did not affect the proportion which implanted. Litters of one or two young, resulting from the transfer of two eggs to one horn only, were successfully carried to late pregnancy and suffered less post-implantational mortality than did larger litters resulting from the transfer of larger numbers of eggs. An empty uterine horn did not exert either a systemic or a local luteolytic effect. Eggs were transferred to one horn of a pseudopregnant female, and at the same time the ovary on that side was removed. Implantation occurred normally, pregnancies were maintained, and the corpora lutea in the remaining ovary did not appear to be affected by the adjacent unoccupied horn. When litters of one or two young were allowed to go to term, the young suffered very heavy perinatal mortality. This was not a direct effect of the small number of young, but was entirely due to the significant prolongation of the gestation period with small litters, combined with the significant decrease in perinatal survival when parturition was delayed.