Proteolysis of vascular basement membranes and surrounding extracellular matrix is a critica1 early step in neovascularization. It requires alteration of the balance between matrix metalloproteinases (MMPs) and proteins that bind to and inactivate MMPs, tissue inhibitors of metalloproteinases (TIMPs). TIMP-1 has been demonstrated to inhibit neovascularization in chick chorioallantoic membranes. However, TIMP-1 has also been shown to either promote or inhibit cell proliferation and migration in different settings. To determine whether genetic alteration of the MMPDIMP-1 ratio would alter retinal neovascularization, we crossed mice that express vascular endothelial growth factor (VEGF) in photoreceptors with TIMP-1-deficient mice or mice that overexpress TIMP-1. Compared to VEGF transgenepositive/TIMP-1-sufficient mice, VEGF transgenepositive1TIMP-1-deficient mice showed smaller neovascular lesions. There was also no difference between the two groups of mice in the appearance of the neovascularization by light or electron microscopy. Compound VEGFITIMP-1 transgenic mice had increased expression of both VEGF and TIMP-1 in the retina, and had more neovascularization than mice that had increased expression of VEGF alone. These gainand loss-of-function data suggest that alteration of the TIMP-1MMP ratio modulates retinal neovascularization in a complex manner and not simply by altering the proteolytic activity and thereby invasiveness of endothelial cells.