A number of significant age-related changes were found to occur in the retinal pigment epithelium (RPE) of normal pigmented ACI rats. Age-related morphological changes in the RPE included: changes in RPE cell height, an increase in lipofuscin content, alterations in the apical microvilli and basal infoldings, and an apparent thickening of the RPE basal lamina. Unusual banded structures were often seen within the basal infoldings of the RPE in senescent but not in young adult rats. Lipid droplets were relatively common in the RPE of senescent rats, but were quite rare in 4- and 11-month-old animals. In addition, contact between the RPE apical microvilli and the photoreceptor outer segments appeared to become less intimate with age. Phagocytosis of rod outer segment discs appeared to be reduced between 4 and 32 months of age. This reduction was greater than could be accounted for solely by a concomitant age-related decrease in the number of photoreceptors per unit retinal length. RPE-choroid acid phosphatase activity (a lysosomal marker), on the other hand, did not appear to change with age. The molar ratio of vitamin A-palmitate to vitamin A-stearate in the PRE-choroid rose significantly with age, from about 1.85 at both 4 and 11 months, to over 2.5 at 28 months. At each age, the palmitate- to stearate-ester ratio was the same in the dark-adapted and light-adapted eyes.