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Retinal capillary quiescence is regulated by a delicate balance between proangiogenic and anti-angiogenic factors. Pathological angiogenesis is the result of a shift in this balance towards proangiogenic influences. Pathological angiogenesis is produced in a rat model of oxygen-induced retinopathy (OIR) by exposing newborn rat pups to alternating periods of hyperoxia and hypoxia. Based upon previous work, two similar exposure paradigms were investigated and compared, exposure of rat pups to alternating periods of 45 and 12.5% oxygen, and to alternating periods of 40 and 15% oxygen. The resulting retinal pathology was assessed by measurement of retinal clock hours with pathological blood vessel growth and the percentage of the retina that is avascular. The 45 and 12.5% exposure produced significantly greater incidence and severity of pathology than the 40 and 15% protocol. To explain the difference in pathology between these two very similar exposure protocols, retinal levels of proangiogenic vascular endothelial growth factor (VEGF) and VEGF receptor 2 (VEGFR2) and anti-angiogenic pigment epithelium-derived factor (PEDF) were measured by ELISA and western blot analysis at 0, 2, and 6 days post-exposure. In whole retinal lysates, there were no significant differences in VEGFR2 and PEDF levels. However, VEGF levels were approximately 48 and 78% higher on post-oxygen exposure day 0 and 2, respectively, in the group treated with alternating periods of 45 and 12.5% oxygen compared to the group treated with alternating periods of 40 and 15% oxygen. There was no significant difference in VEGF levels between these two groups on day 6 post-exposure. Therefore, the difference in pathology observed between these two experimental paradigms is associated with differences in whole retinal VEGF levels, but not changes in whole retinal VEGFR2 or PEDF levels. The results of this study suggest the existence of a threshold in the rat model of OIR, such that a small change in blood oxygen profile triggers a disproportionate increase in subsequent neovascularization, which is accompanied by more dramatic changes of retinal VEGF level than VEGFR2 or PEDF level. If a similar threshold exists for humans, it could explain why some oxygen-treated premature infants develop retinopathy and others do not, despite similar gestational ages, birth weights and clinical courses.