Quercetin is known to protect the cells suffering from oxidative stress. The oxidative stress elevates intracellular Ca2+ concentration, one of the phenomena responsible for cell death. Therefore, we hypothesized that quercetin would protect the cells suffering from overload of intracellular Ca2+. To test the hypothesis, the effects of quercetin on the cells suffering from oxidative stress and intracellular Ca2+ overload were examined by using a flow cytometer with appropriate fluorescence probes (propidium iodide, fluo-3-AM, and annexin V-FITC) and rat thymocytes. The concentrations (1-30 mu M) of quercetin to protect the cells suffering from intracellular Ca2+ overload by A23187, a calcium ionophore, were similar to those for the cells suffering from oxidative stress by H2O2. The cell death respectively induced by H2O2 and A23187 was significantly suppressed by removal of external Ca2+. Furthermore, quercetin greatly delayed the process of Ca2+-dependent cell death although it did not significantly affect the elevation of intracellular Ca2+ concentration by H2O2 and A23187, respectively. It is concluded that quercetin can protect the cells from oxidative injury in spite of increased concentration of intracellular Ca2+. Results suggest that quercetin is also used for protection of cells suffering from overload of intracellular Ca2+. (C) 2008 Elsevier Inc. All rights reserved.