Expression of 72 kDa and 92 kDa type IV collagenases and the metalloproteinase inhibitors TIMPs 1, 2, and 3 was studied by in situ hybridization in implanting mouse embryos of days 5.5 to 7.5. The 92 kDa type IV collagenase was strongly expressed in invading trophoblasts, signals above background not being observed in the embryonic proper or placental tissue. In contrast, signals above background were not seen for the 72 kDa enzyme in any cells of the implantation region, including trophoblasts and stromal cells of the decidual tissue. Only cells in the mucosal stroma outside the decidual region displayed some expression. TIMP-3 was intensily expressed in maternal cells in the area surrounding the invading embryonic tissue. No expression was observed for TIMP-1 or TIMP-2 in the embryo proper, trophoblasts, or the area of the uterine decidual reaction. Weak signals appeared for TIMP-1 only in the circular layer of myometrial smooth muscle and in some uterine stroma cells distant from the site of embryo implantation. The results suggest a central role for 92 kDa type IV collagenase and TIMP-3 in the extracellular proteolysis associated with implantation of the early embryo.