Unilaterally sterilized guinea-pigs were mated and the sterile uterine horn traumatized 0 to 7 days later by the passage of two or more threads which were left in situ. The animals were killed 3 to 7 days after traumatization and the endometrial reactions studied histologically. Although deciduomata developed in uteri traumatized 4 to 7 days after mating, those traumatized up to 3 days after mating and killed up to 5 days later showed little reaction and contrasted markedly with the corresponding horns in which fertilized eggs were becoming implanted. This difference, not affected by exogenous progesterone treatment, indicates that the fertilized eggs have a specific capacity to induce changes in the endometrium, while it is non-reactive to other stimuli.

Traumatization of both uterine horns did not completely inhibit ovoimplantation or subsequent pregnancy.