The aortic endothelium of rabbits fed a stock diet containing 1 or 2% cholesterol for intervals of 1 day to 21 weeks was compared with that of normal rabbits on a stock diet without cholesterol. The endothelium was examined en face using the Häutchen technique. The aortic endothelium of the experimental animals revealed an increase in stigmata and stomata, multinucleated giant cells and leukocytes particularly about sites of branching. These changes were also observed in the endothelium overlying lipid deposits but in addition, there was an increase in mitoses, an alteration in the orientation of endothelial cells and their nuclei, and a change in their argyrophilic staining properties. Closely related to the endothelium was an increasing number of foam cells which appeared to be derived from monocytic leukocytes.