Objective:To directly examine the effects of carnosine on neuronal excitation and inhibition in rat hippocampus in vivo. Methods:Artificial cerebrospinal fluid with carnosine was directly administrated over the exposed rat hippocampus. The changes of neuron activity in the CA1 region of hippocampus were evaluated by orthodromically-and antidromically-evoked potentials,as well as paired-pulse stimulation paradigm. Results:In both orthodromic and antidromic response potentials,carnosine transformed population spikes(PSs) with single spike into epileptiform multiple spikes. In addition,similar to the effect of γ-aminobutyric acidA(GABAA) antagonist picrotoxin,carnosine decreased paired-pulse stimulating depression significantly. However,no significant change was observed in the spontaneous field potentials during the application of carnosine. Conclusion:The results indicate a disinhibition-induced excitation effect of carnosine on the CA1 pyramidal neurons. It provides important information against the application of carnosine as a potential anticonvulsant in clinical treatment. Objective: To directly examine the effects of carnosine on neuronal excitation and inhibition in rat hippocampus in vivo. Methods: Artificial cerebrospinal fluid with carnosine was directly administered over the exposed rat hippocampus. The changes of neuron activity in the CA1 region of hippocampus were evaluated by orthodromically-and antidromically-evoked potentials, as well as paired-pulse stimulation paradigm. Results: In both orthodromic and antidromic response potentials, carnosine transformed population spikes (PSs) with single spike into epileptiform multiple spikes. In addition, similar to the effect of of γ-aminobutyric acid A (GABAA) antagonist picrotoxin, carnosine decreased paired-pulse stimulating depression significantly. However, no significant change was observed in the spontaneous field potentials during the application of carnosine. Conclusion: The results indicate a disinhibition-induced excitation effect of carnosine on the CA1 pyramidal neurons. It provides important informat ion against the application of carnosine as a potential anticonvulsant in clinical treatment.