We have previously reported that shortâ€term treatment of newborn mice with vascular endothelial growth factor (VEGF) receptor tyrosine kinase inhibitors induces abnormal retinal vascular growth and patterns. An exposure of neonatal mice to highâ€concentration oxygen disturbs normal retinal vascular development. The present study aimed to determine (1) whether vascular abnormalities are observed in the retina of newborn mice exposed to high concentrations of oxygen, and (2) how astrocyte network formation is affected following the exposure to hyperoxia. Newborn (postnatal day 0) mice were exposed to 75% oxygen for 48 or 96 hr. During hyperoxia exposure, VEGF expression decreased, and the onset of retinal vascularization was completely suppressed. After completion of the hyperoxic period, retinal vascularization occurred, but it was delayed in a hyperoxic exposure durationâ€dependent manner. In retinas of hyperoxiaâ€exposed mice, dense capillary plexuses were found, and the number of arteries and veins decreased.