The Delta-like ligand 4/Notch signaling pathway was shown to participate in the process of retinal development and angiogenesis.However,the function of the Delta-like ligand 4/Notch signaling pathway in retinopathy of prematurity requires further study.Retinopathy of prematurity was induced in 5-day-old Sprague-Dawley rats exposed to hyperoxia for 7 days,and then returned to room air.Reverse transcription-PCR and western blot revealed that Delta-like ligand 4 levels decreased at postnatal day 12 and increased at postnatal day 17 in retinopathy of prematurity rats.Flat-mounted adenosine diphosphatase stained retina and hematoxylin-eosin stained retinal tissue slices showed that the clock hour scores and the nuclei counts in retinopathy of prematurity rats were significantly different compared to normal control rats.After retinopathy of prematurity rats were intravitreally injected with Delta-like ligand 4 monoclonal antibody to inhibit the Delta-like ligand 4/Notch signaling pathway,there was a significant increase in the severity of retinal neovascularization (clock hours) in the intravitreally injected eyes.The nuclei count was highly correlated with the clock hour score.These results suggest that Delta-like ligand 4/Notch signaling plays an essential role in the process of physiological and pathological angiogenesis in the retina. The Delta-like ligand 4 / Notch signaling pathway was shown in participate in the process of retinal development and angiogenesis. However, the function of the Delta-like ligand 4 / Notch signaling pathway in retinopathy of prematurity requires further study. Retinopathy of prematurity was induced in 5-day-old Sprague-Dawley rats exposed to hyperoxia for 7 days, and then returned to room air. Reverse transcription-PCR and western blot revealed that Delta-like ligand 4 levels decreased at postnatal day 12 and increased at postnatal day 17 in retinopathy of prematurity rats. Flat-mounted adenosine diphosphatase stained retina and hematoxylin-eosin stained retinal tissue sections showed that the clock hour scores and the nuclei counts in retinopathy of prematurity rats were significantly different compared to normal control rats. After retinopathy of prematurity mice were intravitreally injected with Delta-like ligand 4 monoclonal antibody to inhibit the Delta-like ligand 4 / Notch signaling pathway, the re was a significant increase in the severity of retinal neovascularization (clock hours) in the intravitreally injected eyes. The nuclei count was highly correlated with the clock hour score. sese results suggest that delta-like ligand 4 / Notch signaling plays an essential role in the process of physiological and pathological angiogenesis in the retina.