N-methyl-D-aspartate receptors (NMDAR) are heteromeric, cation-selective, ligand- and voltage-gated ion channels with critical roles in both excitatory synaptic transmission and in the induction of many forms of synaptic plasticity, the process by which synaptic weight is dynamically modified in an activity-dependent manner. The principal aim of this project is to resolve the long-standing debate surrounding NMDAR expression and function in the presynaptic axonal compartment, specifically in the induction of spike timing-dependent long-term depression (tLTD) at the layer 4 to layer 2/3 synapse in the mouse somatosensory barrel cortex and the CA3 to CA1 synapse in the hippocampus. To investigate these synapses, whole-cell current clamp recordings of postsynaptic L2/3 or CA1 neurons are made with monopolar stimulation electrodes placed at the base of somatosensory barrels in L4 or in the hippocampal stratum radiatum, respectively. tLTD is induced with 100 pre-before-post pairings, in which a postsynaptic spike is induced 18ms before a presynaptic current pulse. Similarly, tLTP may be induced with a pre-before-post paradigm with presynaptic stimulation 5-10ms before a postsynaptic spike. In acute brain slices prepared from transgenic mice with a Cre-dependent knockout of the obligate NMDAR subunit in either layer 4 (Grin1fx/fx x Six3-Cre) or CA3 (Grin1fx/fx x Grik4-Cre), tLTD cannot be induced with a post-before-pre protocol. A pre-before-post protocol induces robust tLTP in slices from the same animals. In line with previous pharmacological data, these results suggest that the induction of spike timing-dependent long-term depression at L4-L2/3 and CA3-CA1 synapses is dependent upon presynaptic NMDAR function.