Transient myocardial ischemia during cardiac surgery causes a loss of energy sources, contractile depression, and accumulation of metabolites and H⁺ ion resulting in intracellular acidosis. The reperfusion following ischemic cardioplegia recovers intracellular pH, activates Na⁺-H⁺ exchange and Na⁺-Ca²⁺ exchange transports and consequently produces Ca²⁺ overload, which yields cell death. Among the various Ca²⁺ entry pathways, the Na⁺-Ca²⁺ exchanger is known to play one of the major roles during the ischemia/reperfusion of cardioplegia. Consequently, information on the changes in intracellular Ca²⁺ activities of human cardiac myocytes via the Na⁺-Ca²⁺ exchanger is imperative despite previous measurements of Ca²⁺ current of human single myocytes. In this study, human single myocytes were isolated from the cardiac tissues obtained during open-heart surgery and intracellular Ca²⁺ activity was measured with cellular imaging techniques employing fluorescent dyes. We report that the Na⁺-Ca²⁺ exchanger of adult cardiac myocytes is more susceptible to hypoxic insult than that of young patients.