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The current technique was developed to characterize the morphologic changes in the retinas of oxygen-reared rats, as an animal model of retinopathy of prematurity. Past studies have used ink perfusion to observe the retinal vasculature, but this method is static and requires the sacrifice of the subject. Fluorescein angiography, however, is dynamic and relatively noninvasive, and allows the survival of the animal for further study. The fundus camera cannot be used because the source of light that is focused in an annulus is too large for the pupil size of a young (approximately 14-day-old) rat. To overcome this, a Nikon inverted microscope (Diaphot-TMD) was used. Using the proper exciting and barrier filters for fluorescene, a photographic sequence was made by rapidly focusing to the plane of the retinal vessels. To our knowledge, similar photographs have not been previously published. This technique was used in newborn pigmented ratlings that were 1) exposed to 80% oxygen for the first 14 days of life; 2) exposed to 80% oxygen for the first 21 days of life; or 3) exposed for the first 14 days followed by 7 days in room air. Age-matched controls were raised simultaneously in room air and evaluated with the same technique. Differences were observed between treatments in the amount of retinal capillary loss, and in the tortuosity and diameter of the major retinal vessels. The hyaloid system also varied between treatment groups. Oxygen-exposed rats showed a persistence of the hyaloid vessels that was particularly prominent in the group returned to room air before analysis. Comparisons are made to past results obtained with other histologic techniques.