Astrocytes exhibit a form of excitability and communication on the basis of intracellular Ca²⁺ variations (; ) that can be initiated by neuronal activity (; ). A Ca²⁺ elevation in astrocytes induces the release of glutamate (; ; ; ), which evokes a slow inward current in neurons and modulates action potential-evoked synaptic transmission between cultured hippocampal cells , suggesting that astrocytes and neurons may function as a network with bidirectional communication. Here we show that a Ca²⁺ elevation in astrocytes increases the frequency of excitatory as well as inhibitory miniature postsynaptic currents (mPSCs), without modifying their amplitudes. Thapsigargin incubation, microinjection of the Ca²⁺ chelator BAPTA, and photolysis of the Ca²⁺ cage NP-EGTA demonstrate that a Ca²⁺ elevation in astrocytes is both necessary and sufficient to modulate spontaneous transmitter release. This Ca²⁺-dependent release of glutamate from astrocytes enhances mPSC frequency by acting on NMDA glutamate receptors, because it is antagonized by d-2-amino-5-phosphonopentanoic acid (AP5) or extracellular Mg²⁺. These NMDA receptors are located extrasynaptically, because blockage specifically of synaptic NMDA receptors by synaptic activation in the presence of the open channel blocker MK-801 did not impair the AP5-sensitive astrocyte-induced increase of mPSC frequency. Therefore, astrocytes modulate spontaneous excitatory and inhibitory synaptic transmission by increasing the probability of transmitter release via the activation of NMDA receptors.