This study set out to examine how oxidative stress affects cellular energetics in the intact heart under physiologically relevant conditions. Ischemia-reperfusion can be a lethal consequence of treatment for a myocardial infarction as reperfusion is accompanied by arrhythmias in approximately 15% of patients from potentially overwhelming oxidative stress (Bonnemeier 2005). A common problem for diabetics undergoing treatment for a myocardial infarction is acute hyperglycemia which causes additional oxidative stress. To investigate the role of altered glucose in mitochondrial dysfunction, protocols of ischemia-reperfusion injury with and without hyperglycemia were applied to normal intact guinea pig hearts to simulate oxidative stress while measuring mitochondrial membrane potential and electrical activity. High resolution optical mapping of a 4x4mm ² area of myocardium over the left ventricle was conducted whereby the heart was stained with a voltage sensitive probe specific to the inner mitochondrial membrane where the electron transport chain is located. We learned that ischemia results in an organized, steady depolarization of mitochondrial membrane potential (ΔΨm) across the heart and that reperfusion results in a transient re-polarization that is not always sustained. During the course of ischemia-reperfusion injury large spatial heterogeneities and temporal oscillations of ΔΨm are created in hyperglycemic hearts yet both normal as well as hyperglycemic hearts exhibited a comparably high incidence of arrhythmias.