Glial cells synthesise neuroactive substances and release them upon neurotransmitter receptor activation. Homocysteic acid (HCA), an endogenous agonist for glutamatergic N-methyl-d-aspartate (NMDA) receptors, is predominantly localised in glial cells. We have previously demonstrated the release of HCA from mouse astrocytes in culture following activation of β-adrenergic receptors. Moreover, a release of HCA has also been observed in vivo upon physiological stimulation of sensory afferents in the thalamus. Here we report the glutamate-induced release of HCA from astrocytes. The effect of glutamate was mediated by the activation of ionotropic (NMDA and non-NMDA) as well as by metabotropic receptors. In addition, the release of HCA was Ca 2 + - and Na + -dependent, and its mechanism involved the activation of the Na + /Ca 2 + -exchanger. Furthermore, we provide evidence for the presence of functional NMDA receptors on astrocytes, which are coupled to an intracellular Ca 2 + increase via stimulation of the Na + /Ca 2 + -exchanger. Our data thus favour a participation of glial cells in excitatory neurotransmission and corroborate the role of HCA as a ''gliotransmitter.''