Matrix metalloproteinases (MMPs) have been implicated in the formation of microvessels during angiogenesis, but their role in vascular regression is poorly understood. The rat aorta model of angiogenesis was used to study the function of MMPs at different stages of the angiogenic process. Gelatin zymography and Western analysis demonstrated production of MMP-2 and MMP-9 by aortic outgrowths in serum-free collagen gel culture. MMP-2 was found in both culture medium and collagen gel, whereas MMP-9 was predominantly associated with the gel. MMP expression increased gradually during the angiogenic growth phase and stayed high when vessels regressed and collagen lysed around the aortic rings. The MMP inhibitors, batimastat and marimastat, blocked formation of microvessels when added to the culture medium at the beginning of the experiment. They, however, stabilized the microvessels and prevented vascular regression after the angiogenic growth phase. This effect was observed also under conditions of angiogenic stimulation by basic fibroblast growth factor. MMP inhibitor-mediated stabilization of microvessels was associated with inhibition of collagen lysis and accumulation of collagen fibrils in the subendothelial space. This study demonstrates that MMPs promote microvessel formation during the early stages of angiogenesis, but also contribute to the reabsorption of the neovasculature in the later stages of this process. The time-dependent divergent effects of MMPs on microvessel growth and survival may influence the in vivo activity of MMP inhibitors used to treat angiogenesis-dependent disorders.