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Rearing neonatal rats in hyperoxia induces the development of retinal hemorrhages and retinal dysplasia. Albino rats were placed in 80% oxygen immediately after birth and were exposed for either 5, 10, or 14 days, followed by sacrifice or exposure to normoxia for an additional 2, 4, 5, 7, 8, 10, 38, 45 or 56 days. Control rats were simultaneously raised in room air and sacrificed at the same times. All animals were enucleated and their eyes processed for light and electron microscopy. Eyecups were trimmed to facilitate cross-sectioning of the retina in the vertical meridian. No control rats showed signs of retinal hemorrhages or of dysplastic folds or rosettes. Nor did the retinas of rats killed immediately after oxygen exposure contain hemorrhages, but the incidence of retinal folds or rosettes in this group was 54%. For rats exposed to combinations of hyperoxia and brief normoxia (10 days or less), 40% suffered hemorrhages and 50% developed retinal folds or rosettes. Although hemorrhages were more prominent in rats subjected to longer periods of oxygen (73% of all rats exposed for 14 days followed by brief normoxia vs. 6% of those exposed for 5 days followed by brief normoxia), the incidence decreased with time post-exposure in room air. Hemorrhages occurred in 100% of the rats raised in oxygen for 14 days followed by 2 days in room air, and decreased to 50% by 7 days in room air and to 0% by 38 days, indicating a spontaneous resolution with time. In each case, the blood appeared to leak from the newly-forming vessels of the deep capillary net, with most of the red blood cells migrating to the subretinal space. Retinal fold or rosette formation, indicative of developmental dysplasia, occurred in a fraction of virtually all groups of exposed rats, and persisted at the longest post-exposure periods. These two manifestations of oxygen-induced retinopathy are emphasized because they lead to an abnormal separation of the retina from the epithelial layer, which may increase the likelihood of the most serious consequence of ROP--retinal detachment. In fact, all rats that endured post-exposure periods of 38 days or longer before sacrifice exhibited retinal detachment.