The rationale for this study was to determine whether in the hypercholesterolemic rabbit any evidence of endothelial injury could be detected prior to or during the early phase of fatty intimal lesion formation. The data presented showed that in the first 12 weeks of feeding a 0.1% cholesterol-rich diet, rabbit aortas were covered with an intact endothelium. Focal areas of increased endothelial cell replication were observed adjacent to the aortic ostia at 12 and 20 weeks. These replicating cells were almost exclusively located at the shoulders of large raised lesions. In a similar fashion, adherent leukocytes were observed adjacent to the aortic ostia, and at later times they were concentrated at the periphery of these intimal lesions. Smooth muscle cell replication, as assessed by autoradiography, was found to be significantly increased only after 20 weeks of feeding the lipid-rich diet. These data suggest that an increased endothelial cell turnover and leukocyte adhesion were the first detectable changes induced by cholesterol feeding and that smooth muscle cell proliferation occurred soon after these events.