Objective: The protective effect of estrogen against early atherosclerosis in animal models is well documented, but the mechanisms responsible for this effect are not well understood. The earliest recognizable event in the pathogenesis of atherosclerosis is an increased recruitment of macrophages into the arterial subendothelium. Macrophages first play a protective role by removing low-density lipoproteins, but when the cholesterol is in excess, macrophages are converted into foam cells and form atheromas. Recent human and animal data indicate that the recruitment of macrophages to the arterial wall is mediated by monocyte chemotactic protein-1 (MCP-1). We hypothesized that one of the mechanisms of estrogen's protective effect against atherosclerosis may be the down-regulation of MCP-1 expression in the arterial wall.