Acute ischemia is a well-known inductor of extracellular matrix (ECM) remodeling, which leads to the development of congestive heart failure and is associated with left ventricular dilatation. Here we investigate the timecourse of ECM processing with release of endostatin (ES) and other low-molecular-weight fragments during early ischemia-reperfusion of the heart. In this blinded study, 30 pigs were randomized to 60 min of global myocardial ischemia at either 4 or 37°C or served as control. Five transmyocardial tissue samples were collected at baseline and after ischemia within 150 min of reperfusion. Collagen XVIII cleavage products of 10–75 kDa including ES (25 kDa) were analyzed using the Western blot and ELISA method, and creatin kinase as marker of myocardial injury was determined in samples collected from the coronary sinus. We demonstrate that processing of the extracellular matrix protein collagen XVIII starts during early reperfusion, as we observed a significantly increased expression of cleavage products at 10 and 75 kDa as well as ES at 150 min of normothermic ischemia-reperfusion. We further demonstrate a differential processing of collagen XVIII depending on temperature conditions during myocardial ischemia, as an increase in cleavage products was observed after normothermic ischemia only; however, expression of ES and other fragments remained unchanged after hypothermic ischemia-reperfusion and in controls. In conclusion, this blinded study first demonstrated that processing of extracellular matrix started early after ischemia-reperfusion and depends on temperature conditions. These findings may contribute to a broader understanding of matrix processing after ischemia-reperfusion.