Glutamate is the established transmitter at most brain synapses. Recent evidence has shown that glutamate can be released from astrocytes. Here we combine electron microscopic immunocytochemistry, live imaging techniques and patch clamp electrophysiology to show that astrocytes contain a vesicular compartment competent of glutamate release and that this astrocyte derived glutamate can increase the activity of adjacent excitatory synapses by activation of presynaptic NMDA 2B receptors. Activation of astrocytic P2Y1 receptors elicits astrocytic glutamate release and the astrocyte dependant changes in synaptic activity during neuronal activity.