Valproic acid and meropenam is commonly co-administrated in neurosurgical patients. Meropenem potentially decreases the valproic acid level, which may cause perioperative seizure. There has been no previous report of drug interaction between valproic acid and meropenem in Thailand. The authors report a patient who faced uncontrolled seizure after co-administrated valproic acid and meropenem. The level of valproic acid was assessed in different times after the administration of meropenem. Rapid decrease of valproic level was detected. However due to the administration of other antiepileptic agents, seizure did not develop. It is important for the physicians to recognize drug interaction between valproic acid and meropenem. Avoiding co-administration of both agents, valproic acid level monitoring and additive of other antiepileptic agents seem to be the appropriate solution.