Aspartate belongs together with glutamate to the main excitatory transmitters in CNS. This aminoacid is a potent agonist of NMDA-subtype glutamate receptors, densely expressed in hippocampal synapses. Since these receptors are crucial for synaptic plasticity under physiological and pathophysiological conditions, it is important to obtain more insight in the regulation of aspartate release. Although the fact that aspartate release from presynaptic terminals has been widely recognized, the precise origin of the amino acid, i.e. vesicular or cytosolic, is still a matter of debate. In addition, aspartate is an important intermediate in neuronal energy metabolism. Recent in vivo and in vitro findings from our laboratory favour the view that the major source of aspartate released from nerve terminals is the cytosolic pool. From this cytosolic pool aspartate can be released by reversed operation of the aspartate/glutamate transporter. This reversed transport is regulated by a presynaptic NMDA autoreceptor. Such a regulation depends on the extrasynaptic glutamate concentration, and may contribute to cross-talk between neighbouring synapses.