Na(+)/Ca(2+) exchanger (NCX), by mediating Na(+) and Ca(2+) fluxes bi-directionally, assumes a role in controlling the Ca(2+) homeostasis in the ischemic brain. It has been suggested that the three isoforms of NCX (NCX1, 2 and 3) may be differentially involved in permanent cerebral ischemia. However, the role of NCX2 has not been defined in ischemic reperfusion injury after a transient focal cerebral ischemia. Furthermore, it is not known whether NCX2 imports or exports intracellular Ca(2+) ([Ca(2+)](i)) following ischemia and reperfusion. To define the role of NCX2 in ischemia and reperfusion, we examined mice lacking NCX2, in vivo and in vitro. After an in vitro ischemia, a significantly slower recovery in population spike amplitudes, a sustained elevation of [Ca(2+)](i) and an increased membrane depolarization were developed in the NCX2-deficient hippocampus. Moreover, a transient focal cerebral ischemia in vivo produced a larger infarction and more cell death in the NCX2-deficient mouse brain. In particular, in the wild type brain, NCX2-expressing neurons were largely spared from cell death after ischemia. Our results suggest that NCX2 exports Ca(2+) in ischemia and thus protects neuronal cells from death by reducing [Ca(2+)](i) in the adult mouse brain.