Increased aminoaciduria in metachromatic leucodystrophy is demonstrated, presumably for the first time. By means of circular and two-dimensional chromatography, the free amino acids of urine, blood, saliva, and cerebrospinal fluid of two familial cases of juvenile metachromatic leucodystrophy were studied. In blood an increased concentration of cystine, aspartic acid, glycine, glutamic acid, alanin, proline, tyrosine, and phenylalanin was found. In urine, cystine, aspartic acid, alanin, proline, tyrosine, and phenylalanin showed increased levels. In saliva a-y-diaminobutyric acid, glutamic acid, alanin, and proline were mostly concentrated. In cerebrospinal fluid aspartic acid, alanin, proline, and phenylalanin were found in greater concentrations. Besides, 6 non-identified amino acids were present. One of them disintegrated into 3 components and one of these fractions behaved like a polypeptide, seemingly identifiable to a compound substance of glutamic acid, glycine, and aspartic acid. This substance could integrate a polypeptid-lipid complex resulting from a lipid dysmetabolism, already known in the leucodystrophies, allied to a proteid dysmetabolism, now reported. The results are compared with the data reported in the literature regarding the concentrations of amino acids in the cerebrospinal fluid and urine of some nervous diseases showing demyelinating processes. In CSF the pattern of metachromatic leucodystrophy resembles those previously found in Niemann-Pick disease, cerebromacular degeneration, and "diffuse sclerosis". In urine the pattern resembles that of Hartnup disease.