Membrane type matrix metalloproteinases (MT-MMPs) possess a C-terminal transmembrane domain and are expressed on the cell membrane. It was suspected, therefore, that MT1-MMP might play an important role in the trophoblastic invasion during implantation. The patterns of expression and localization of membrane type matrix metalloproteinase-1 (MT1-MMP) were examined immunocytochemically in cultured mouse blastocysts and excised extoplacental cones (EPCs). MT1-MMP immuno-reactivity was present in the giant trophoblast cells located at the periphery of the spreading trophoblast of cultured blastocysts and the outgrowths of cultured EPCs, but not in the densely packed trophoblast cells in both the blastocysts and the EPCs. It appears likely that MT1-MMP expressed on the edge of the invading trophoblast facilitates the trophoblastic invasion by cleaving proMMP-2, a known substrate of MT1-MMP, in the decidua. Immunohistochemical examination of early conceptuses confirmed that the trophoblast cells actively invading the endometrium in vivo express MT1-MMP strongly. It is suggested, furthermore, that the expression of MT1-MMP might be downregulated by cell-cell contact in mouse trophoblast cells, as in the mouse mammary epithelial cell line HC11.