The permeability of retinal blood vessels to intravenously injected horseradish peroxidase was examined two to seven days after bilateral common carotid ligation in rats. Light and electron microscopy revealed leakage from the blood vessels of the optic disc. This leakage appeared to result from vesicular transport across the endothelium; the interendothelial tight junctions we examined remained intact and there was no evidence of endothelial cell death. The intraretinal blood vessels and the retinal pigment epithelium retained their normal impermeability to horseradish peroxidase. Cellular edema was present at the optic disc, manifested as swollen axons. Glial cells remained normal in appearance and the extracellular space was not enlarged. These observations clarify those previously derived from funduscopy and fluorescein angiography, which revealed disc edema and hyperfluorescence.