The overload of glutamate transporters by excessive extracellular glutamate leads to excitotoxic cell death in multiple neurodegenerative diseases as well as stroke, hypoxia and traumatic brain injury. The sustained activation of extrasynaptic N-methyl-D-aspartate (NMDA) receptors by excess extracellular glutamate has been implicated in multiple disease models in vitro and in vivo. Pathological levels of extracellular glutamate arise from compromised or even reversed function of excitatory amino acid transports (EAATs) in a brain region and subtype dependent manner. We have used whole cell patch clamp of hippocampal CA1 pyramidal neurons in acute brain slices from adult mice (P70-91) to assess synaptic and extrasynaptic NMDA receptor function in the presence EAAT blockers and/or elevated glutamate levels (10-100 µM). Both DL-threo-β-Benzyloxyaspartic acid (DL-TBOA, 30 µM) and TFB-TBOA (3 µM) evoked large tonic currents mediated by NMDA receptors and occluded synaptic NMDA-receptor mediated excitatory postsynaptic currents (EPSCs) similar to the effects of bath perfusion with glutamate (10 µM). Such effects are likely due to reverse activation of the transporters. Lower concentrations of TFB-TBOA (100 nM) which do not block EAAT3, or the selective EAAT1 or EAAT2 blockers (UCPH101 and dihydrokainate respectively) had little effect. The lack of a recognised selective blocker for EAAT3 makes its contribution difficult to assess. These results identify the importance of EAATs in buffering excess glutamate and maintaining synaptic and minimizing extrasynaptic NMDA receptor activation in a physiological ex-vivo slice model from mature mice.