The physical law of diffusion imposes O2 concentration gradients from the plasma membrane to the center of the cell. The present study was undertaken to determine how such intracellular radial gradients of O2 affect the fate of isolated single cardiomyocytes. In single rat cardiomyocytes, mitochondrial respiration was moderately elevated by an oxidative phosphorylation uncoupler to augment the intracellular O2 gradient. At physiological extracellular O2 levels (2-5%), decreases in myoglobin O2 saturation and increases in NADH fluorescence at the center of the cell were imaged (anoxic cell core) while the mitochondrial membrane potential (DeltaPsim) and ATP levels at the anoxic cell core were relatively sustained. In contrast, treatment with 0.5 mM iodoacetamide (IA) to inhibit creatine kinase (CK) resulted in depletion of both DeltaPsim and ATP at the anoxic cell core. Even at normal extracellular Po2, actively respiring cardiomyocytes developed rigor contracture followed by necrotic cell death. Furthermore, such rigor was remarkably accelerated by IA, whereas cell injury was perfectly rescued by mitochondrial F1Fo inhibition by oligomycin. These results suggest that increases in radial gradients of O2 potentially promote cell death through the reverse action of F1Fo in mitochondria located at the anoxic cell core. However, in the intact cardiomyocyte, the CK-mediated energy flux from the subsarcolemmal space may sustain DeltaPsim at the cell core, thus avoiding uncontrolled consumption of ATP that can lead to necrotic cell death. Mitochondria at the anoxic core can cause necrotic cell death in cardiomyocytes at physiological extracellular Po2.