AIM:To investigate the effect of angiotensin converting enzyme inhibitors(ACEI)on suppressing retinal neovascularization(RNV).METHODS:It was a random control experimental study.Thirty-six seven-day-old C57BL/6J mice were divided into two groups randomly including hyperoxia group and ACEI group,eighteen in each group.ACEI group and hyperoxia group were exposed to 75% oxygen to establish a model of oxygen-induced retinopathy,and received daily intraperitoneal injections of captopril 10mg/kg and the equal amount of saline respectively for five days continuously after leaving oxygen box.Fluorescent angiography was used to assess the vascular pattern of retina.The proliferative neovascular response was quantified by counting the nuclei of new vessels extending from the retina into the vitreous in cross-sections.Angiotensin II(Ang II)and vascular endothelial growth factor(VEGF)protein levels in retinas were measured by Western blot.RESULTS:Fluorescent angiography presented increasing neovascular tufts with fluorescein leakage in hyperoxia group.Neovascular tufts and fluorescein leakage were decreased in ACEI group compared to the hyperoxia group.The number of endothelial cells of new vessels extending from retina to vitreous were decreased significantly in ACEI group compared with hyperoxia group(P=0.00).The expression of Ang II protein in retinas were decreased significantly in ACEI group compared with hyperoxia group(P=0.00).The expression of VEGF protein in retinas were decreased significantly in ACEI group compared with hyperoxia group(P=0.00).CONCLUSION:Early treatment with ACEI could inhibit RNV to some degree and protect the retinas.