Since previous studies have suggested the involvement of the angiotensin system in the aging-related endothelial dysfunction [10], [17], the expression level of angiotensin II, AT1 receptors, AT2 receptors and angiotensin-converting enzyme (ACE) was assessed by immunohistochemical staining in the mesenteric artery of young and middle-aged rats. The gene discussed is ACE; the disease is endothelial dysfunction.