Several molecules have been shown to be involved in glial-neuronal communication, including S100B, an astrocyte-derived neurotrophic cytokine. Extracellular S100B protects hippocampal neurons from excitotoxic damage, whilst toxic levels of glutamate to neurons have been shown to reduce S100B secretion in astrocytes and brain slices, by an unknown mechanism. Here, we investigate which mechanisms are possibly involved in this effect in primary cultures of hippocampal astrocytes using glutamate agonists and glutamate uptake inhibitors. DCG-IV, an agonist of group II metabotropic glutamate receptors, caused a smaller decrease in S100B secretion when compared to 1 mM glutamate. D: -aspartate partially reverted the glutamate effect on S100B release and two other inhibitors, PDC and DIDS, reverted it completely. These findings suggest that S100B secretion is inversely coupled to glutamate uptake. Decrease in S100B secretion may be considered as direct excitotoxic damage, but a beneficial mechanism effect cannot be ruled out, because S100B elevation could cause an additional cell death.