Long-term potentiation (LTP) in the hippocampal CA1 region requires the activation of NMDA receptors (NMDAR). NMDAR activation in turn requires membrane depolarization as well as the binding of glutamate and its coagonist, glycine. Previous pharmacological studies suggest that the glycine transporter type 1 (GlyT1) maintains subsaturating concentrations of glycine at synaptic NMDAR. Antagonists of GlyT1 increase levels of glycine in the synaptic cleft and, like direct glycine site agonists, can augment NMDAR currents and NMDAR mediated functions such as LTP. In addition, stimulation of the glycine site initiates signaling through the NMDAR complex, priming the receptors for clathrin-dependent endocytosis. We have used a new potent GlyT1 antagonist, CP-802,079 with whole-cell patch-clamp recordings in acute rat hippocampal slices to determine the effect of GlyT1 blockade on LTP. Reverse microdialysis experiments in the hippocampus of awake, freely moving rats, showed that this drug elevated only the extracellular concentration of glycine. We found that CP-802,079, sarcosine and glycine significantly increased the amplitude of the NMDAR currents and LTP. In contrast, application of higher concentrations of CP-802,079 and glycine slightly reduced NMDAR currents and did not increase LTP. Overall, these data suggest that the level of glycine present in the synaptic cleft tightly regulates the NMDAR activity. This level is kept below the ''set point'' of the NMDAR internalization priming mechanism by the presence of GlyT1 dependent uptake.