Accumulation of monocyte-derived foam cells in focal areas of the arterial intima is a key step in early atherogenesis. We investigated the expression of vascular cell adhesion molecule-1 (VCAM-1), a mononuclear leukocyte adhesion molecule, in the arterial endothelium during the early phases of diet-induced atherogenesis in rabbits in vivo and the regulation of VCAM-1 expression by cytokines in rabbit aortic organoid cultures in vitro. Rabbits were fed either an atherogenic diet (containing 0.3% cholesterol, 9% coconut oil, and 1% corn oil) or an isocaloric control diet (10% corn oil) for 4 days or 1, 3, 6, or 13 weeks. The endothelium in the ascending aorta focally expressed VCAM-1 after only 1 week on the atherogenic diet but before the first appearance of intimal macrophages, as judged by immunohistochemical staining of serial sections. The rabbits that consumed the atherogenic diet for 3 weeks or longer developed lesions in the intima composed of macrophages bearing class II major histocompatibility antigen (MHC-II). Endothelial cells continued to focally express VCAM-1 at sites of MHC-H-positive intimal macrophages for up to 13 weeks. The ascending aortas of control rabbits lacked VCAM-1 or MHC-II-positive endothelium or macrophages at all times studied. These observations demonstrate the focal activation of arterial endothelium as early as 7 days after initiation of an atherogenic diet (at serum cholesterol levels of 308 ±57 mg/dl). In organoid cultures of rabbit thoracic aortas, Gram-negative bacterial lipopolysaccharide or the cytokines interleukin (IL)-lcx, tumor necrosis factor-*, and IL-4, as well as interferon gamma, induced VCAM-1 expression in the endothelium. Our results suggest that the activation of cytokine-inducible endothelial functions, such as expression of VCAM-1, may participate in the initiation of diet-induced atherosclerosis in rabbits.