Objective To explore the effect of oxygen inhalation on the retinae of newborn rats and its mechanism.Methods We mimicked the retinopathy of prematurity(ROP) by putting the newborn rats in high concentrated oxygen. One-day old rats were put into the oxygen box with the oxygen concentration of 80% for continuous 7 days; then in air condition for 7 days. The arterial blood oxygen pressure, retinal superoxide dismutase (SOD), and malondialdehyde (MDA) of the rats (1,2,4,7,8,9,11,14 days old) were examined. The diameter of retinal vessels′main branch and the coverage rate of peripheral vessels were measured in 7- and 14-day-old rats by ink perfusion. The retinal neovascularization of rats (8,9,11, 14 days old) were observed by HE staining. The rats of the same age fed in air condition were in the control group.Results The differential pressures of blood oxygen of rats (1,2,4,7 days old) in study group were significantly higher than those in the control group (P＜0.01), while the differential pressures of blood oxygen of rats (8,9,11,14 days old) in study group were lower than those in the control group (P＞0.05). The contents of SOD of the retinae in the rats ( 1,2,4,7,8 days old) were significantly lower than those in the control group(P＜0.01, P＜0.05 ), while the contents of MDA were significantly higher than those in the control group (P＜0.01,P＜0.05). The diameter of retinal vessels′main branch in 7-day rats was 75% of the control group, and the coverage rate of peripheral vessels was 22% of the control group; and was 61% and 73% respectively in 14-day-old rats. The neovascularization could be seen in 16.7% of the rats in the study group and nought in the control group.Conclusion The damage of free radical of the retina in high concentrated oxygen and hypoxia situation after oxygen supply may be one of the most important mechanism of ROP. (Chin J Ocul Fundus Dis，2003，19：269-332)