Reperfusion by Ca2+-containing solution after 10 minutes of Ca2+-depletion resulted in the loss of myoglobin, depletion of ATP and phosphocreatine (calcium paradox). Exhaustion of high energy phosphates exacerbated when glucose was substituted by pyruvate or acetate. Iodoacetate (30 mcM) elevated loss of myoglobin due to calcium paradox. Addition of 120 mM glucose reduced loss of myoglobin and depletion of high energy phosphates after induction of calcium paradox. There is a close correlation between concentration of lactate and ATP in the hearts subjected to calcium paradox. Elevated activity of glycolysis protected herats against the calcium paradox.