Background: Mast cells are multifunctional cells containing various mediators such as cytokines, tryptase, and histamine and they have been identified in infarct myocardium. We investigated the function of mast cells in angiogenesis, inflammatory process, and adaptation of cardiac regeneration pathways in an acute myocardial infarction (AMI) rat model. Methods: Mast cell granules (MCGs) were prepared from purified mast cells, which was isolated from rat peritoneal fluid, by stimulation with compound 48/80. The physiological and functional roles of MCGs were evaluated on endothelial cells, rat neonatal cardiomyocytes, and infarct heart (one hour of the occlusion of left anterior descending coronary artery followed by reperfusion). Results: The number of mast cells had two peak time point of appearance into infarct region at one day and 21 days after MI induction in a rat (p<0.01 in each compared with sham operated heart). Simultaneous injection of optimal dose MCGs with MI induction resulted in an increased macrophage infiltration (CD68+ cells) and decreased apoptosis of myocardium without change of mast cell number in infarct myocardium at 14 days after MI. Moreover, MCGs injection attenuated the progression of MI through angiogenesis and preservation of left ventricular function [left ventricular end-diastolic pressure and dp/dt (-)] at 14 days after MI. MCGs administration also shifted the c-kit expression dominantly to the cytoplasm with nucleus expression at one day after MI. MCGs-treated cardiomyocytes were more resistant to hypoxic injury through phosphorylation of Akt (Thr-308). MCGs-treated endothelial cells showed enhanced migration and tube formation (p<0.05 in each). Conclusion: We revealed the novel cardioprotective roles of MCGs on MI via prolonged survival of cardiomyocytes and induction of angiogenesis. Our results provide new insights into the function of mast cells in the early pathophysiologic environment after MI.