Simultaneous recordings of the time course of change in the extracellular glutamate concentration ([Glu]O) and intracellular Ca2+ level [Ca2+]i during glucose deprivation were performed in the CA3 area of hippocampal slices from guinea pig. Direct measurement of [Glu]O with a glutamate sensitive sensor revealed that [Glu]0 showed a biphasic increase during glucose deprivation and decreased rapidly after re-introduction of glucose in the perfusing medium. The initial increase of [Glu]O preceded [Ca2+]i increase and thereafter a sustained increase of [Glu]O was observed concomitant with massive [Ca2+]i accumulation. Replacement of Na with Li markedly attenuated the rapid reduction of [Glu]O, suggesting that the reduction of [Glu]O was due to Na-dependent glutamate uptake system, which worked immediately after energy recovery.