The authors have determined sodium and potassium contents of normal cerebrospinal fluid and the range of concentrations was from 140 to 150 mEq/1 for sodium, and for potassium it was from 2.3 to 3.4 mEq/1. These determinations were performed with the flamme photometer with internal lithium standard. When studying disorders of electrolyte and water metabolism in diseases of the central nervous system, they have observed 99 patients and determined both sodium an potassium contents in cerebrospinal fluid. In cases with hipo-osmolarity they have observed that the cerebrospinal fluid potassium content was in the range of normality generally. In cases of hyperosmolarity they have observed that 2/3 of the cases were in the range of normality, but in the remaining cases there was a little increase in the potassium content, and the greatest figure was 4.3 mEq/1. The results point out to the great value of the determination of cerebrospinal fluid sodium content in the early recognition of disorders of electrolyte and water metabolism in diseases of the central nervous system. On the other hand, the determination of cerebrospinal fluid potassium content has no practical value in the recognition of potassium metabolism disorders in the body.