Infiltration of immunologically active cells into vein grafts is concomitant with the development of intimal hyperplasia (IH) and often leads to obliterative stenosis and graft failure. Previous work has demonstrated the prolonged presence of monocytes and macrophages in vein grafts. The stimuli attracting these macrophages remain unidentified. Monocyte chemotactic protein-1 (MCP-1), a potent and specific chemokine for monocytes/macrophages, is secreted by smooth muscle cells, endothelial cells, fibroblasts, and leukocytes, all of which are present in grafted veins. In this study, we examined the temporal profile of MCP-1 gene expression in rat vein grafts by using reverse transcription-polymerase chain reaction (PCR) and immunohistochemistry. Epigastric vein-to-femoral artery bypass grafts were microsurgically placed and harvested at various time points after grafting. Histological analysis confirmed the consistent development of IH. PCR was performed and relative levels of MCP-1 quantified by autoradiography. Our results show that MCP-1 mRNA levels increased 28-fold by 4 hours after grafting and up to 117-fold by 1 week. After this time MCP-1 mRNA levels decreased; nonetheless, even at 8 weeks after grafting, message levels remained elevated 7-fold above baseline. Immunoreactive MCP-1 protein and ED1+ macrophages were detected at all time points; the degree of immunostaining correlated with MCP-1 mRNA levels. Our results support the hypothesis that upregulation of MCP-1 gene expression in vein grafts results in the recruitment of monocytes and tissue macrophages to the vein wall, which leads to IH. The correlation between monocyte/ macrophage infiltration and IH suggests a critical role for these cells in IH development.