The migration, proliferation, and neointimal accumulation of vascular smooth muscle cells (VSMCs) are key events in the development and progression of many vascular diseases and a predictable consequence of mechanical injury to the blood vessel. VSMCs in vivo are surrounded by and embedded in extracellular matrices (ECMs) that must be traversed during migration. In many other cell types, migration across ECM barriers involves the local destruction or degradation of these barriers by extracellular proteases. Principle among such proteases are those belonging to the matrix metalloproteinase (MMP) family. Using an in vitro assay to monitor and manipulate the ability of VSMCs to degrade a defined ECM barrier as they migrate toward a chemoattractant, we demonstrate the VSMCs isolated from the rat thoracic aorta and maintained in a proliferating or "synthetic" state readily migrate through an ECM barrier of reconstituted basement membrane. The migration of serum-starved/differentiated VSMCs toward the chemoattractant both in the presence and in the absence of the barrier is less than 20% (p less than 0.001) that of proliferating cells. The importance of MMP expression during the migration of "synthetic" VSMCs through the reconstituted BM was demonstrated using a peptide that mimics the inhibitory propeptide region of all MMPs. This peptide blocked migration of proliferating cells through the barrier by more than 80% (p less than 0.005), but did not significantly affect migration that occurred in the absence of the barrier. Likewise, antisera capable of neutralizing the activity of the 72 kD type IV collagenase (MMP-2) also inhibited migration through the barrier, without significantly affecting the migration of cells in the absence of the barrier. Northern blotting and zymogramic analyses indicate that MMP2 is the principal MMP expressed and secreted by these cells. MMP2 activity expressed by serum starved/differentiated VSMCs as measured by a fluorescent peptide cleavage assay was less than 5% of that measured in proliferating VSMCs. These results demonstrate that VSMCs migrate through an ECM barrier similar in composition to one that normally surrounds them and that this ability is regulated by the phenotypic state of the cell.