NEUROTRANSMITTER released from neurons is known to signal to neighbouring neurons and glia1-3 Here we demonstrate an additional signalling pathway in which glutamate is released from astrocytes and causes an NMDA (N-methyl-d-aspartate) receptor-mediated increase in neuronal calcium. Internal calcium was elevated and glutamate release stimulated by application of the neuro-ligand bradykinin to cultured astrocytes. Elevation of astrocyte internal calcium was also sufficient to induce glutamate release. To determine whether this released glutamate signals to neurons, we studied astrocyte-neuron co-cultures. Bradykinin significantly increased calcium levels in neurons co-cultured with astrocytes, but not in solitary neurons. The glutamate receptor antagonists d-2-amino-5-phosphonopentanoic acid and d-glutamylglycine prevented bradykinin-induced neuronal calcium elevation. When single astrocytes were directly stimulated to increase internal calcium and release glutamate, calcium levels of adjacent neurons were increased; this increase could be blocked by d-glutamylglycine. Thus, astrocytes regulate neuronal calcium levels through the calcium-dependent release of glutamate.