The intravenous sedation with propofol is very useful for patients with mental retardation and challenging behavior. However, it is very difficult to titrate the dose of propofol for maintaining the adequate sedation depth because of the difficulty of verbal communication with them. Most of them have a epilepsy and take antiepileptic drugs. Antiepileptic drugs are thought to influence on the effect of anesthetics. Valproate is reported to inhibit the metabolism of propofol. Therefore, we divide the patients into two groups : patients receiving valproate (valproate group), patients receiving no antiepileptic drugs (control group), and compare the required dose of propofol and the recovery time.