The CSF fluoride level of individuals drinking water with normal fluoride content and of patients with endemic fluorosis were studied. For the purpose of studying the relationship between the dynamic equilibrium of the CSF fluoride and other body fluids, urine and blood fluoride were examined simultaneously. Fluoride was revealed in every CSF sample of the control group and its mean value was lower than that of the blood. The CSF fluoride concentration of patients with fluorosis was slightly higher than that of the control group, although there was no statistically significant difference. The results suggests that fluoride is a normal component of CSF. In severe cases of fluorosis or breakdown of the blood-brain in some diseases of the central nervous system, the CSF fluoride value might be increased.