The Na+/Ca2+ exchanger has not been characterized in rat mast cells, although its existence has been previously suggested. In this work, we determine that this exchanger exists on rat mast cells and that it has an important regulatory role on the cellular function. We have studied uptake and release of 45Ca in the presence of different external sodium concentrations and, under the same conditions, the simultaneous uptake of 22Na and 45Ca. The results show that uptake and release of 45Ca in these cells are related to the concentration of sodium in the extracellular medium and that there is also a perfect coupling between 22Na and 45Ca fluxes. In these conditions, we evaluated the intracellular calcium levels in fura-2 loaded cells. When the extracellular sodium concentration was lower than 60 mM, we observed an increase in intracellular calcium, reaching its maximum when the extracellular medium has no sodium. Then we investigated the effect on histamine release. The ionophore A23187 elicits histamine release in rat mast cells, depending on the extracellular calcium concentration. This drug releases more histamine (up to twofold) with sodium concentrations <60 mM. In the presence of 2,4-dichlorobenzamil hydrochloride, a Na+/Ca2+ exchanger inhibitor, the release of histamine induced by the ionophore was lower than in controls in media with low external sodium, and, on the contrary, at extracellular sodium concentrations >60 mM, the histamine release was higher than in controls. In the same conditions, but when the Na+-K+ ATPase was inhibited by ouabain, and as a consequence more sodium was inside the cells, a high increase in histamine release induced by A23187 in a sodium-free medium was observed. Under the same conditions, a high increase in intracellular calcium takes place. The overall data are preliminary evidence suggesting the existence of a Na+/Ca2+ exchanger in rat mast cells with a threshold to get reversed at 60 mM external sodium, lower concentrations of this ion increasing internal calcium and producing higher histamine release.