Early reperfusion of an ischemic region can result in significant salvage of the area at risk. We show the presence of hydroxyl free radicals at the time of post ischemia reperfusion using electron paramagnetic resonance (EPR) spectroscopy in a macaque model. These free radicals may be formed as a result of reperfusion or may be an un-involved bystander. It is possible that they may be involved in reperfusion injury.