Long-term potentiation (LTP) of synaptic transmission provides an experimental model for studying mechanisms of memory 1. The classical form of LTP relies on N-methyl- D-aspartate receptors (NMDARs), and it has emerged that astroglia can regulate their activation through Ca 2+-dependent release of the NMDAR co-agonist D-serine 2- 4. Release of D-serine from glia enables LTP in cultures 5 and explains a correlation between glial coverage of synapses and LTP in the supraoptic nucleus 4. However, Ca 2+ elevations in astroglia can also release other signalling molecules, most prominently glutamate 6- 8, Adenosine-5′-triphosphate 9, and Tumor-Necrosis-Factor-α 10, 11 whereas neurons themselves can synthesise and supply D-serine 12, 13. Furthermore, loading an astrocyte with exogenous Ca 2+ buffers does not suppress LTP in hippocampal area CA1 14- 16, and the physiological relevance of experiments in cultures or strong exogenous stimuli applied to astrocytes has been questioned 17, 18. The involvement of glia in LTP induction thus remains controversial. Here we show that clamping internal Ca 2+ in individual CA1 astrocytes blocks LTP induction at nearby excitatory synapses by reducing the occupancy of the NMDAR co-agonist sites. This LTP blockade can be reversed by exogenous D-serine or glycine whereas depletion of D-serine or disruption of exocytosis in an individual astrocyte blocks local LTP. We thus demonstrate that Ca 2+-dependent release of D-serine from an astrocyte controls NMDAR-dependent plasticity in many thousands of excitatory synapses occurring nearby.