Myocardial infarction is associated with the rapid induction of mononuclear cell chemoattractants promoting monocyte infiltration into the injured area. Monocyte to macrophage differentiation and macrophage proliferation allow a long survival of monocytic cells, critical for effective healing of the infarct. In a canine infarction/reperfusion model, newly recruited myeloid leukocytes were markedly augmented during early reperfusion (5-72 hours). By seven days, the number of newly recruited myeloid cells was reduced and the majority of the inflammatory cells remaining in the infarct were mature macrophages. Macrophage-Colony Stimulating Factor (M-CSF) is known to facilitate monocyte survival, monocyte to macrophage conversion, and macrophage proliferation. We demonstrated marked induction of M-CSF mRNA in ischemic segments persisting for at least five days after reperfusion. M-CSF expression was predominantly localized to mature macrophages infiltrating the infarcted myocardium; the expression of the M-CSF receptor, c-fms, a protein with tyrosine kinase activity was found in these macrophages but was also observed in a subset of microvessels within the infarct. A significant number of infarct macrophages expressed Proliferating Cell Nuclear Antigen (PCNA), a marker of proliferative activity. In vitro M-CSF induced Monocyte Chemoattractant Protein (MCP)-1 synthesis in canine venous endothelial cells. M-CSF-induced endothelial MCP-1 upregulation was inhibited by herbimycin A, a tyrosine kinase inhibitor and by LY-294002, a phosphatidylinositol 3'-kinase (PI3K) inhibitor. We suggest that upregulation of M-CSF in the infarcted myocardium may have an active role in healing not only through its effects on cells of monocyte/macrophage lineage, but also by regulating endothelial cell chemokine expression.