OBJECTIVE: To define the role of fms-like tyrosine kinase 3 (Flt3) in the heart. BACKGROUND: Flt3 is a prominent target of receptor tyrosine kinase inhibitors (TKIs) used for anticancer therapy. TKIs can cause cardiomyopathy, but understanding of the mechanisms is incomplete, partly because the roles of specific TKI target receptors in the heart are still obscure. METHODS: Myocardial infarction was induced in mice by permanent ligation of the left anterior descending coronary artery followed by intramyocardial injection of Flt3 ligand (FL) or vehicle into the infarct border zone. Cardiac morphology and function were assessed by echocardiography and histology one week after infarction. In addition, Flt3 expression and regulation as well as molecular mechanisms of Flt3 action were examined in cardiomyocytes in vitro. RESULTS: The intramyocardial injection of FL into the infarct border zone decreased infarct size and ameliorated post-myocardial infarction remodeling and function in mice. This beneficial effect was associated with reduced apoptosis including myocytes in the infarct border zone. Cardiomyocytes expressed functional Flt3, and Flt3 mRNA and protein were upregulated under oxidative stress, identifying cardiomyocytes as FL target cells. Flt3 activation with FL protected cardiomyocytes from oxidative stress-induced apoptosis via an Akt-dependent mechanism involving Bcl-2 family protein regulation and inhibition of the mitochondrial death pathway. CONCLUSION: Flt3 is a cytoprotective system in the heart and a potential therapeutic target in ischemic cardiac injury. The protective mechanisms uncovered here may be further explored in view of potential cardiotoxic effects of Flt3-targeting anticancer therapy, particularly in patients with ischemic heart disease.