The ciliary beating function of ependymal cells covering the ventricular surface of the brain plays an important role in the flow of cerebrospinal fluid. The aim of the present study is to know whether mouse ependymal ciliary movement is inhibited by the administration of methylmercury (MeHg) included in drinking water to mouse. Ciliary beat frequency (CBF) was measured during 30 minutes on brain slices that included the third ventricle. When MeHg water was drunk by mice for 19 days, the CBF decreased dose-dependently with MeHg, and a significant decrease of CBF was observed in both 20 mg/L and 40 mg/L MeHg-administered groups. Next, we examined how many days after mice consecutively drank 20 mg/L of MeHg water the CBF decreased. CBF decreased significantly after the 4th day of MeHg drinking. After mouse was drunk MeHg water for 19 days, the drinking water without MeHg was changed and the CBF was observed up to 30 days. CBF recovered after 20 days. In addition, there was a relationship between the decrease of CBF and the increase of brain mercury content. From these results, it is concluded that continuous MeHg intake causes a decrease of CBF on ependymal cells and the decrease of CBF gradually recovers after MeHg intake is stopped. It is suggested that CBF decreases as the amount of MeHg in brain tissue increases.