One of the main steps in the elucidation of the mechanism of ovum implantation is to determine how the blastocyst stimulates the uterus to undergo a decidual reaction. The extent of this reaction varies between species but is particularly marked in rodents, and normal ovum implantation does not occur without it. It has been known for several years, however, that the uterus of a suitably primed rodent can be stimulated to produce decidual cells in the absence of a blastocyst simply by traumatization of the uterus. The so-called deciduoma which results is thought to be analogous to the decidual reaction of normal implantation because (a) it is histologically similar (Krehbiel, 1937), and (b) the hormonal requirements for its formation are similar to those for implantation. Thus the deciduoma is a useful model on which to investigate factors which may be important in the initiation of the decidual response. It is also of interest as an example of extremely rapid cellular proliferation (the uterus may increase its weight four-fold in 72 hours) and so may provide information about cellular growth in general. If the deciduoma is analogous to the decidual reaction it is a reasonable assumption that the mechanisms by which the two responses are initiated have at least a final stage in common.