Breakdown in the blood-retinal barrier occurs in retinal neovascularization in a number of diseases. To study the anatomic basis of this breakdown, we examined retinal neovascularization induced by injection of 250,000 homologous fibroblasts into the vitreous cavity of pigmented rabbits. Neovascularization is evident by electron microscopy in this model 3 days after fibroblast injection. Fluorescein angiography followed by intravenous horseradish peroxidase (HRP) injection was performed prior to enucleation on 2, 3, 5, 7, and 14 days after fibroblast injection. Fluorescein leakage from retinal vessels occurs early (at day 1) and persists as the neovascularization progresses. The leakage in the early stages is concentrated near puckers from the medullary wings. In the later stages, fluorescein leakage is most prominent in the developing tips of the new vessels. Horseradish peroxidase was not observed to leak from the lumen of new vessels. "Gaps" or separations in the endothelial cell junctions were not observed in developing vessels. The breakdown of the blood-retinal barrier in this model of retinal neovascularization is therefore selective, (ie, fluorescein leaks but not HRP) and it is not due to gaps or fenestrations between endothelial cells in developing vessels.