Calcium influx associated with the opening of NMDA receptor channels is the major signal triggering synaptic and developmental plasticity. Controlling the NMDA receptor function is, therefore, critical for many functions of the brain. We explored the mechanisms of synaptic activation of the NMDAR glycine site by endogenous coagonist using whole-cell voltage-clamp recordings from hippocampal neurons in mixed cultures, containing both neurons and glial cells, and, under more physiological conditions, in hippocampal slices. Here, we show that the glycine site of the NMDA receptor at hippocampal synapses, both in culture and acute brain slices, is not saturated by the ambient coagonist concentration and is modulated through activity-dependent coagonist release. Augmentation of the NMDA receptor-mediated synaptic responses by local glutamate-induced coagonist release is spatially restricted and determined by spatiotemporal summation of synaptic events at neighboring synaptic inputs on a single dendritic branch. Therefore, different spatiotemporal patterns of presynaptic activity could be translated into different levels of the NMDAR activation in specific afferent projections. These results suggest that the NMDA receptor glycine site may serve as a detector of the spatiotemporal characteristics of presynaptic activity patterns.