OBJECTIVE Angiotensin II is critically involved in left ventricular remodeling after myocardial infarction. Neovascularization has been thought to prevent the development of left ventricular remodeling and deterioration to heart failure. To elucidate the role of angiotensin II in neovascularization during cardiac remodeling, we induced myocardial infarction in angiotensin II type 1a receptor (AT1) knockout (KO) mice. METHODS AND RESULTS There were more vessels in the border zone of infarcted hearts of wild-type (WT) mice and AT1KO mice at 14 days after operation, compared with in the left ventricle of sham-operated mice, and the number was larger in WT mice than in AT1KO mice. Consistent with these observations, the infarcted heart of AT1KO mice expressed lower levels of matrix metalloproteinase and endothelial nitric oxide synthase activity. More inflammatory cells such as granulocytes and macrophages were infiltrated in the infarcted hearts of WT mice than AT1KO mice at 4 days. A variety of cytokines and chemokines were increased in infarcted hearts of WT and AT1KO mice, and many of them were more remarkable in WT mice than in AT1KO mice at 14 days. CONCLUSIONS AT1 plays a critical role in inflammatory cell infiltration, cytokine production, and neovascularization in infarcted hearts.