Epiblast cells in the early chick embryo differentiate to form all three germ layers through ingression of cells at the primitive streak across the basement membrane that underlies the epiblast. We tested the idea that degradation of the extracellular matrix components by matrix metalloproteinase(s) (MMPs) is involved in this process. Epiblast cells and primitive streak cells were dissociated into single cells and seeded onto a reconstituted basement membrane gel in vitro. Following overnight culture, approximately half the cells made holes in the substratum by dissolving the gel matrix. This invasive phenomenon was reproduced in vitro even when the cells were cultured upside down using a hanging culture system. We detected gelatinase activity in the culture supernatants from both prestreak epiblast cells and primitive streak cells. Pro-MMP-2 was detected in the culture media of the prestreak/streak cells as a 72-kDa band by gelatin zymography. In RT-PCR experiments, mRNAs for MMP-2, membrane-type (MT)3-MMP and MMP-11(stromelysin-3) were expressed in the epiblast cells before and during primitive streak formation. Injection of GM 6001 or other MMP inhibitors into the subgerminal cavity of the embryo inhibited the formation of the primitive streak and/or the primitive groove in more than 82% of the injected embryos. On the other hand, injection of a negative control compound instead of GM 6001 did not cause substantial inhibition. These results suggest that MMPs are involved in the enzymatic degradation of the basement membrane underlying the epiblast and are thus important for the ingression of mesendodermal cells along the primitive streak.