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Retinopathy of prematurity, a disease characterised by aberrant retinal vascular development in premature neonates, is a leading cause of blindness and visual impairment in childhood. This work sought to examine differences in the susceptibility of inbred rat strains to oxygen-induced retinopathy, a model of human retinopathy of prematurity. The overriding aim was to identify genetic factors in rats that might be generalisable to humans. Newborn rats of six different strains were exposed to alternating cycles of hyperoxia and relative hypoxia for fourteen days. Rats were removed to room air and killed for analysis immediately, to assess oxygen-induced retinal vascular attenuation, or four days later to evaluate the extent of hypoxia-induced vasoproliferation. Whole flat-mounted retinae were stained with fluorophore conjugated isolectin GS-IB4, and measurement of vascular area was conducted using fluorescence microscopy and video-image analysis. A hierarchy of susceptibility to the inhibitory effects of cyclic hyperoxia and relative hypoxia on postnatal retinal vascularization was identified for the rat strains studied. Susceptibility to vascular attenuation was predictive of the subsequent risk of vascular morphological abnormalities. Cross-breeding experiments between susceptible and resistant strains demonstrated that the susceptible phenotype was dominantly inherited in an autosomal fashion. These studies confirmed an association between ocular pigmentation and retinopathy risk, however the finding of differential susceptibility amongst albino rat strains implicated factors in addition to those associated with ocular pigmentation. Quantitative real-time reverse transcription-polymerase chain reaction was used to compare the retinal expression of angiogenic factor genes in susceptible and resistant strains with the aim of identifying a genetic basis for the strain difference. Eight angiogenic factor genes were selected for study: vascular endothelial growth factor (VEGF); VEGF receptor 2; angiopoietin 2; Tie2; pigment epithelium-derived factor; erythropoietin; cyclooxygenase-2 and insulin-like growth factor-1. The most notable difference between strains was the expression of vascular endothelial growth factor (VEGF) during the cyclic hyperoxia exposure period - higher VEGF expression was associated with relative resistance to retinopathy. Other differences in retinal angiogenic factor gene expression between strains, such as higher expression of VEGF receptor 2 and angiopoietin 2 in resistant strains, appeared to be secondary to those in VEGF. Following cyclic hyperoxia, the expression pattern of angiogenic factor genes changed - messenger RNA levels of hypoxia-induced genes, including VEGF, VEGF receptor 2, angiopoietin 2 and erythropoietin, were significantly higher in those strains with larger avascular areas, than in those strains that were relatively resistant to retinopathy. These findings provide firm evidence for hereditary risk factors for oxygen-induced retinopathy in the rat. Differences in the regulatory effects of oxygen on VEGF expression appear to be central to the risk of retinopathy. The potential relevance of these hereditary factors is discussed in the context of the human disease.