Objective:　To　research　the　effect　of　melatonin　against　glutamate　excitotoxicity.　Methods:　The　model　of　glutamate-induced　excitotoxic　damage　was　built　up　in　rat　cerebral　cortical　cell　culture.　The　effect　of　melatonin　against　excitotoxic　injury　was　observed　by　determining　the　leakage　rate　of　lactate　dehydrogenase(LDH)　from　neurons.　Results:　The　leakage　rate　of　LDH　wasn＇t　decreased　markedly　when　cultures　were　exposed　to　melatonin　before,　during　or　6　h　after　glutamate　treatment.　The　leakage　rate　of　LDH　was　decreased　significantly　when　melatonin　was　administered　0　h,　2　h　or　4　h　after　the　cultures　were　exposed　to　glutamate.　The　inhibitory　function　of　melatonin　on　LDH　leakage　was　most　effective　at　2　h　and　4　h.　Conclusion:　Melatonin　has　protective　effects　on　neurons　damaged　by　glutamate　in　a　certain　time　limit.