Trophoblast cell invasion into the uterine wall is characteristic of hemochorial placentation. In this report, we examine trophoblast cell invasion in the rat and mouse, the endocrine phenotype of invasive trophoblast cells, and aspects of the regulation of trophoblast cell invasion. In the rat, trophoblast cells exhibit extensive interstitial and endovascular invasion. Trophoblast cells penetrate through the decidua and well into the metrial gland, where they form intimate associations with the vasculature. Trophoblast cell invasion in the mouse is primarily interstitial and is restricted to the mesometrial decidua. Both interstitial and endovascular rat trophoblast cells synthesize a unique set of prolactin (PRL)-like hormones/cytokines, PRL-like protein-A (PLP-A), PLP-L, and PLP-M. Invading mouse trophoblast cells also possess endocrine activities, including the expression of PLP-M and PLP-N. The trafficking of natural killer (NK) cells and trophoblast cells within the mesometrial uterus is reciprocal in both the rat and mouse. As NK cells disappear from the mesometrial compartment, a subpopulation of trophoblast cells exit the chorioallantoic placenta and enter the decidua. Furthermore, the onset of interstitial trophoblast cell invasion is accelerated in mice with a genetic deficiency of NK cells, Tg epsilon 26 mice, implicating a possible regulatory role of NK cells in trophoblast cell invasion. Additionally, the NK cell product, interferon-gamma (IFNgamma), inhibits trophoblast cell outgrowth, and trophoblast cell invasion is accelerated in mice with a genetic deficiency in the IFNgamma or the IFNgamma receptor. In summary, trophoblast cells invade the uterine wall during the last week of gestation in the rat and mouse and possess a unique endocrine phenotype, and factors present in the uterine mesometrial compartment modulate their invasive behavior.