The diverse strategies adopted among species for the maintenance of luteal function converge to meet the indispensable requirement for this hormone particularly at the time of onset of implantation. What has emerged from immunization studies is the difference that exists between various species in the effects of progesterone depletion. The findings affirm the uterus as a primary site of progesterone action in the preimplantation period of gestation in the mouse, whereas in the hamster the positive feedback by progesterone on pituitary luteotrophin secretion is important in maintaining luteal function and hormone secretion at a level necessary for uterine preparation. The impact of progesterone in the rat seems to be established within 48 h after fertilization, whereas in the ferret the hormone's action ensures a suitable uterine environment in which the early embryo can flourish. Its effects in early pregnancy in the marmoset are ambiguous from the present studies unless it emerges that target organ responses are tuned to low concentrations of active steroid. However, the discoveries in mice of a conserved family of immunoglobulin genes used exclusively by immunogenic forms of progesterone conjugated to proteins to stimulate antibody production, and of antibody binding to the uterine epithelium, reveal systems potentially inimical for embryo survival. The endometrial expression of the proto-oncogene erb-A at a time when the embryo has not yet arrived in the uterus, and of antibody-binding to the uterine epithelium, are early maternal responses whose significance require closer examination. The present findings support the hypothesis that the primary site of action of progesterone during the preimplantation period differs between species, and that it is not only the mother that recognizes, but the embryo that initiates early changes before the onset of implantation 'in the bed of soil that nourishes it', a symbiosis that may yet prove to be a common feature of the adoption of viviparity as a preferred mode of reproduction.