The morphology of the retinal pigment epithelium (RPE), Bruch's membrane (complexus basalis), choriocapillaris and tapetum lucidum has been studied in the eye of the ranch mink (Mustela vison) by light and electron microscopy. The RPE is composed of a single layer of cells joined laterally by apically located junctional complexes. Basally (sclerally) these cells display numerous infoldings whereas apically (vitreally) two types of processes are associated with rod and cone outer segments. Smooth endoplasmic reticulum and mitochondria are abundant in these cells whereas rough endoplasmic reticulum and polysomes, although present, are not plentiful. An occasional wandering phagocyte is noted at the RPE-photoreceptor interface. In the posterosuperior part of the fundus, a degenerative tapetum lucidum is present. The presence of only a few layers of tapetal cells containing but little reflective material and the haphazard arrangement of this material makes it very unlikely that this area functions as an effective tapetum lucidum. The RPE over the aberrant tapetum, however, shows the morphology that is seen when a functioning tapetum cellulosum is present, namely the absence of melanosomes and an indented choriocapillaris. Bruch's membrane in non-tapetal areas is pentalaminate but, over the tapetum and where it is associated with capillary profiles, it is reduced to a single, thickened basal lamina. The choriocapillary endothelium is highly fenestrated and in nontapetal areas these capillaries are not indented into the epithelial layer.