Ultrastructural changes in the retinal pigment epithelium (RPE) and adjacent photoreceptor cells have been followed in the Wistar rat during the course of long-term vitamin A deficiency. Of particular interest has been the discovery of unusual concentric aggregates within the photoreceptor outer segments and the inner cytoplasm of the RPE. The aggregates were present throughout the course of the retinal degeneration induced by vitamin A deficiency and could be identified in the RPE either by themselves in the apical cytoplasm or within phagolysosomes. It is postulated that the concentric aggregates result initially from abnormal formation or condensation of outer segment membranes and are then slowly degraded by lysosomal action in the RPE cytoplasm. In addition, acid phosphatase activity and typical phagosomes (shed outer segment packets) have been demonstrated in the cytoplasm of the RPE of the vitamin A deficient rats. The latter findings indicate that, at least to some extent, the normal phagocytic and lytic processes of the RPE are retained in this nutritional disorder.