A form of retinal degeneration in the Wistar-Furth strain of rat is described. The changes observed was classified as either mild or severe, depending on the severity of the dystrophy seen by light and electron microscopy. In the mild stage, there was a slight decrease in the number of photoreceptor nuclei in the outer nuclear layer (ONL) and there were pyknotic nuclei in the inner nuclear layer. A few photoreceptor outer and inner segments showed focal swellings. The basal infoldings of the retinal pigment epithelium (RPE) were distributed irregularly, creating spaces that contained collagen-like fibrils. However, the interepithelial tight junctions remained impermeable to intravenously injected peroxidase. In severely dystrophic retinas, there was a further decrease in the width of the ONL, and the photoreceptor outer and inner segments were degenerated. In extreme cases, the photoreceptor cell disappeared completely and the inner retina abutted the RPE. Further changes were also noted in the basal membrane of the RPE; in some regions the membrane was totally flat while in others it formed exaggerated infoldings. In addition, the RPE was frequently vascularized by vessels that originated from the retina. The endothelium of those intra-RPE vessels located near Bruch's membrane was frequently fenestrated. Alterations also occurred in the retinal capillaries; some were degenerated, while others showed increased tortuosity or focal thickenings of the basal lamina. In addition, there were focal increases in the amount of stroma between Bruch's membrane and the choriocapillaris.