Myocardial infarction is associated with an acute inflammatory response, leading to replacement of injured cardiomyocytes with granulation tissue. Mast cells are actively involved in postinfarction inflammation by releasing histamine and tumor necrosis factor-alpha, triggering a cytokine cascade. During the proliferative phase of healing, mast cells accumulate in the infarct and may regulate fibrous tissue deposition and angiogenesis by releasing growth factors, angiogenic mediators, and proteases. This chapter describes simple and reliable methods used to identify mast cells in control and infarcted canine hearts. Toluidine blue staining, labeling with conjugated avidin, and tryptase histochemistry are useful in the detection of mast cells in canine tissues. In the healing infarct, mast cells are associated with other cell types that are important for granulation tissue formation. We present immunohistochemical methods identifying monocytes, neutrophils, macrophages, endothelial cells, myofibroblasts, and smooth muscle cells in dog infarcts. These techniques are useful tools for pathological studies in canine models.