Fluoride is found in the atmosphere, water, soil, coal, food, dental and industrial uses. There were some case reports concerning acute fluoride poisoning in workplaces and laboratories. However, there is limited information concerning the mechanism of fluoride-induced cell death. To study the cytotoxicity of fluoride, the effect of sodium fluoride (NaF) on rat thymocytes has been examined by using a flow cytometer with appropriate fluorescence probes for membrane and cellular parameters. The cytotoxicity of NaF under nominal Ca2+-free condition was significantly lower than that under control condition. NaF also increased intracellular Ca2+ concentration. NaF significantly increased the population of shrunken cells and the cells positive to annexin V. Both are known to be parameters for early stage of apoptosis. However, NaF decreased the population of cells with hypodiploidal DNA, indicating that NaF apparently attenuated spontaneous apoptosis in rat thymocytes. It may be suggested that NaF induces necrosis, associated with some apoptotic characteristics.