Excessive release of L-glutamic acid (glu) has been associated with seizures and epilepsy. Some microdialysis studies have demonstrated an increase in glu levels during seizures both in human and in different animal models of experimental epilepsy. With these techniques it is difficult to monitor the glu concentrations with sufficient time resolution to clearly associate them with EEG activity. To solve this, we have built an electrochemical biosensor based on H2O2 production. A glu biosensor was inserted in the hippocampus of rats with an attached isolated tungsten wire to simultaneously record epileptiform EEG activity. 4-Aminopyridine (10 nmol) was administered into the entorhinal cortex to induce seizures. EEG activity and glu concentrations were measured in real time in awake rats through the use of a swivel to capture and digitize analogical signals. When the first epileptiform burst appeared, it was accompanied by a single and significant increase in glu that could play an essential role in the initiation of the seizure. Subsequent and lesser glu increases also were observed; however they were not directly correlated with further bursts it could be relevant to maintenance of seizures. Sustained increase in glu concentration associated with a flat EEG recording was present when rats died.