Chromatographic analysis of the amino acid composition of cerebrospinal fluid (CSF) protein hydrolysate has revealed significant changes in two diseases of the nervous system. The percentage composition of serine and valine were found to be significantly elevated in the CSF of five patients with subacute sclerosing panencephalitis, while the percentage composition of arginine, glycine, and histidine were higher, and that of leucine lower, in two patients with metachromatic leukodystrophy. No published data concerning the amino acid composition of CSF protein appears to exist. The significance of these alterations remains to be elucidated.