Retinas isolated from a 50/10 oxygen (inspired oxygen cycled between 50% oxygen and 10% oxygen every 24 hours)-induced rat model of retinopathy of prematurity (50/10 OIR model), and from room air-raised rat pups (RA) at birth, age 14 days (persistent peripheral avascular retina in the 50/10 OIR model and complete retinal vascularization in RA) and age 18 days (intravitreous neovascularization in the 50/10 OIR model) were analyzed for messenger RNA of VEGF(164), neuropilin 1, neuropilin 2, VEGF receptor 1, VEGF receptor 2, pigment epithelium-derived factor, and pigment epithelium-derived factor receptor by real-time polymerase chain reaction.