Abstract: Objective To investigate the role of calcium/calmodulin-dependent protein kinase II (CaMKII) in myocardial ischemia-reperfusion (IR) injury in isolated perfused rat heart and explore the underlying mechanisms. Methods An ischemiareperfusion (IR) model was prepared using isolated rat hearts perfused with Krebs-Henseleit solution were randomly divided into control group, 2.5 μmol/L KN-93 group, IR (induced by ischemia for 45 min followed by reperfusion for 120 min) group and KN-93 + IR group. The myocardial performance was evaluated by assessing the left ventricular pressure. Lactate dehydrogenase (LDH) activity and cTnI content in the coronary flow and the infarct size were determined to evaluate the myocardial injury. The phosphorylation of CaMKII (p-CaMKII) and PLN (p-PLN) and oxidation of CaMKII (ox-CaMKII) were measured with Western blotting. The activity of mitochondrial superoxide dismutase (SOD) and the content of malondialdehyde (MDA) were determined using ELISA. Results Compared with the control group, KN-93 treatment at 2.5 μmol/L produced no significant effects on cardiac function or performance in rat hearts without IR injury. Myocardial IR injury significantly decreased myocardial performance and mitochondrial SOD activity in the perfused hearts (P<0.01) and caused significantly increased infarct size, LDH activity, cTnI content, expressions of p-CaMKII, ox-CaMKII and p-PLN, and also increased mitochondrial MDA content (P<0.01). KN-93 treatment at 2.5 μmol/L administered before ischemia and before reperfusion markedly attenuated such changes induced by ischemia and reperfusion (P<0.01). Conclusion CaMKII participates in myocardial IR injury in isolated rat heart, and inhibiting CaMKII can alleviate myocardial injury by relieving mitochondrial oxidation stress.