Rodent photoreceptor dystrophies are characterized by late stage ingrowth of retinal blood vessels into the retinal pigment epithelium (RPE) where they proliferate. Some of these vessels develop the fenestrated phenotype of the choriocapillaris (CC). To determine if development of fenestrae in these endothelial cells is a function of the duration of time the endothelial cell had been encapsulated by the RPE, we did an ultrastructural morphometric study of these vessels in urethane induced photoreceptor degeneration in Long-Evans rats. Retinas of animals aged 20, 24, 40 and 56 weeks were studied.The fraction of vessel profiles within the RPE that had fenestrated endothelial cells increased from 10% to 90% between 20 to 56 weeks. The average number of fenestrae per vessel increased approximately 25 fold between 20 and 24 weeks but stabilized after that, despite a decrease in the number of vessels present at 56 weeks. A large number of degenerated retinal vessel profiles were seen in the RPE at 40 weeks. These facts support the idea that the presence of the RPE induces endothelial cell fenestrae, and also show that a complex process of remodelling including proliferation and degeneration is occurring in these vessels. Analogies between the basic cell biology of neovascularization occuring in these rodent models and that of proliferative diabetic retinopathy and age-related macular degeneration are discussed.