Calcium and magnesium have been measured in cerebrospinal fluid by atomic absorption spectrophotometry in children. The normal values on 194 C.S.F., obtaining for the calcium x: 5.24 mg. % and s: +/- 0.378 mg. % [50--56 % lower than serum values] and for magnesium x: 2.64 mg. % and s: +/- 0.155 mg. % [19--33 % higher than serum values] are found. Higher values of calcium at birth and on the first year of life and no differences with magnesium are noted. Applying the t-test, between normal values obtained and the different pathological entities, authors find singificant differences on the level of calcium, finding higher values on the following diseases: dehydration by diarrhoea, poliomyelitis, anoxy, tumours, bacterial meningitis. Magnesium showed values significantly higher in dehydration by diarrhoea and epilepsy, and values significantly lower on febrile convulsions and virical and bacterial meningitis.