The ultrastructure of the aortic intima and serum lipid levels in Fischer 344 rats were examined at the ages of 12, 18 and 24 months after feeding the animals an atherogenic diet (2% cholesterol, 0.25% sodium cholate, 5% beef fat) for 6 months. Structural changes in the intima were noticeable only at the age of 24 months. In control rats, the endothelial cells were irregular in shape and each had a well-developed Golgi complex and a few lipid droplets. Simultaneously, reticular, basal lamina-like material and electron-dense granules of extracellular liposomes accumulated in the subendothelium. In fat-fed rats, these structural changes were more conspicuous in association with hypercholesterolemia, and numerous monocytes with lipid droplets were attached to the endothelium, occasionally invading into the subendothelium. Slight foam cell lesions were evident in the intima. The finding that older rats were more susceptible to the atherogenic diet suggests that atherogenesis in the rat is promoted by intrinsic age-associated changes in the aortic intima.