The osmolality of the cerebrospinal fluid (CSF) in normal conditions was evaluated through the results in 22 CSF samples obtained from patients with any evidence of central nervous system diseases as well with any disease that might interfer with general metabolic conditions. For comparative purposes it was studied the serum osmolality in a series of 22 healthy people. The osmolality was evaluated by the Fiske's osmometer. Results are presented in table 1 (CSF), table 2 (blood sera) and their estimatives are grouped in table 3. The results reported confirm that there are any significative differences between the values of CSF and of blood serum osmolality in normal conditions.