To test the belief that blood is toxic to the retina, we developed an experimental model of vitreous hemorrhage in rabbits by injecting various amounts of autologous, uncoagulated , whole blood into their vitreous cavities, with the vitreous humor either intact or previously compressed by an expanding perfluoropropane gas bubble. Blood in the eyes with compressed vitreous cleared in half the time required for the same volume injected into eyes with intact vitreous (75.6 days vs 39.2 days). Large volumes of intravitreal blood (0.25 to 0.50 ml) were never toxic to the retina by ganzfeld and bright-flash electroretinography throughout a four-month observation period. The results indicated that massive vitreous hemorrhage has a dense filtering effect that can extinguish the ganzfeld but not the bright-flash electroretinogram. Blood caused moderate hemoglobin staining of the retina, without significant iron staining (confirmed by X-ray microprobe analysis). Hemoglobin residues accumulated within cells of the inner retina, especially Müller's cells. Blood clot retraction after the injection of large volumes of fresh blood (1 ml) produced traction retinal detachment, hole formation, and subretinal accumulation of blood. This correlated with a complete and persistent extinction of the ganzfeld and bright-flash electroretinograms throughout the four-month observation period. A striking finding was that almost all eyes developed glial membranes on the peripheral retina. Fibrous membranes, causing local retinal contraction, were found over the medullary wings and optic disk in eyes with vitreous compression.