Free amino acids were determined quantitatively in cerebrospinal fluid of 80 boys and 69 girls (ages 1 month-14years). On the basis of the cliniclal criterions these children were considered to be ""healthy''. The total number of examined children was 2000. Cerebrospinal fluid was mixed with an equal volume of 5% sulphosalicylic acid and the amino acids were separated by ion exchange column chromatography. RESULTS 1. The sexspecific investigation of the 1 month -2 years old children showed that the boys had a statistical significant hgher concentration of lysine and that no statistical significant differences existed between the groups of 2-14 years old children. 2. The age specific comparison revealed that the amino acid concentrations ofaspartic acid, citrulline, histidine, lysine, serine, threonine and tyrosine were in the cerebrospinal fluid of the younger boys statistical significant higher as compared to the older boys and that the concentration of serine in the cerebrospinal fluid of the younger girls was statistical significant higher than in the group of older girls.