Myocardial infarction is associated with the rapid induction of mononuclear cell chemoattractants that promote 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 h). By 7 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 (MCSF) is known to facilitate monocyte survival, monocyte-to-macrophage conversion, and macrophage proliferation. We demonstrated marked induction of MCSF mRNA in ischemic segments persisting for at least 5 days after reperfusion. MCSF expression was predominantly localized to mature macrophages infiltrating the infarcted myocardium; the expression of the MCSF 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. Many infarct macrophages expressed proliferating cell nuclear antigen, a marker of proliferative activity. In vitro MCSF induced monocyte chemoattractant protein-1 synthesis in canine venous endothelial cells. MCSF-induced endothelial monocyte chemoattractant protein-1 upregulation was inhibited by herbimycin A, a tyrosine kinase inhibitor, and by LY-294002, a phosphatidylinositol 3′-kinase inhibitor. We suggest that upregulation of MCSF 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.