Aortic tissues of cholesterol-fed African green monkeys were studied by light and electron microscopy. The sampled areas were either grossly normal, or showed minute grayish white translucent or yellowish white intimal elevations. Samples of topographically corresponding areas from animals on normal diet served as controls. By light microscopy, the intimal elevations were either pure fatty dots or a combination of fatty dots and focal intimal edema. The most conspicuous ultrastructural findings in the experimental animals were a marked increase in the number of Weibel-Palade bodies, and figures suggestive of release of their content into the vascular lumen. Invaginations or pseudochannels of varying depth as well as intracytoplasmic lipid inclusions were also observed in the experimental series. These findings suggest that Weibel-Palade bodies may play a role in the initial stages of atherosclerotic lesions, and that transendothelial transport may take place via transendothelial channels.