Objective Nitric oxide (NO) production and Ca 2+ homeostasis are key determinants for the control of many cell functions. NO is known to be a mediator of Ca 2+ homeostasis in a highly complex and cell-specific manner and although Ca 2+ homeostasis has been explored in human oral cancer cells, the exact mechanisms are not completely understood. In this study we investigated the impact of exogenous NO on [Ca 2+ ] c homeostasis in PE/CA-PJ15 cells. Design Cells were treated with S-nitrosocysteine as NO-donor and the determinations of cytosolic Ca 2+ concentrations were performed using FURA-2 AM. Carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) and oligomycin were used to challenge mitochondrial functionality, whereas thapsigargin (TG) and La 3+ were employed to perturb intracellular calcium levels. Results NO derived from S-nitrosocysteine (CySNO) induced a dose-dependent reduction of cytosolic calcium [Ca 2+ ] c whereas oxy-haemoglobin (oxyHb) completely counteracted this effect. Subsequently, we assessed possible relationships between NO and cellular structures responsible for Ca 2+ homeostasis. We found that uncoupling of mitochondrial respiration with carbonyl-cyanide-4-(trifluoromethoxy)-phenylhydrazone (FCCP) and oligomycin strongly reduced the effect of NO on [Ca 2+ ] c . Moreover, we found that during this mitochondrial energetic deficit, the effect of NO on [Ca 2+ ] c was also reduced in the presence of La 3+ or thapsigargin. Conclusions NO induces a concentration-dependent [Ca 2+ ] c reduction in PE/CA-PJ15 human oral cancer cells and potentiates mitochondrial Ca 2+ buffering in the presence of TG or La 3+ . Further, we show that exogenous NO deregulates Ca 2+ homeostasis in PE/CA-PJ15 cells with fully energized mitochondria. [ABSTRACT FROM AUTHOR]