After Reiss, Hemphill, the author estimated the cholinesterase values (Ch E) of the spinal fluid in normal infants and of those with disease of the nervous system by Warburg's manometer. As the result, it has been found that the Ch E of the spinal fluid in the nervous system disease fluctuates variously according to diseases, and that the greater is the acuteness of the disturbances, the higher is the tendency of the rise in the Ch E values. 1. The average Ch E value of the spinal fluid in 36 cases of normal infants ranging in age from six months to 15 years is 11.5±1.1 (the fluctuation range being within 7.3 to 16.0) and it rises along with age. The mutual index between the Ch E of the spinal fluid and age is 0.88, showing a strong mutual relationship; but no difference by sex can be recognized 2. The Ch E values in 14 cases with polyradiculoneuritis showing the so-called Guillain-Barre syndrome are elevated, especially high around 10 to 30 days after the onset of disease, and the fluctuation coincides well with the symptoms of posterior radicular irritation. In Heine-Medin disease the Ch E is normal or slightly elevated, but it is clearly lower than that in the Guillain-Barre syndrome, showing significant differences between 10 and 20 days or 20 and 30 days respectively after the onset of disease with 5 per cent margin of error. 3. In Guillain-Barre syndrome and Heine-Medin disease the mutual indices between the Ch E value of the spinal fluid and the protein content are 0.84 and 0.61 respectively, demonstrating the mutual relationship. Moreover, the mutual indices between the Ch E of the spinal fluid and the cell count in both diseases are only 0.01 and 0.004 respectively. 4. In viral diseases with exception of herpangina which shows no disturbances of the nervous system, Ch E values in Japanese encephalitis, serous menignitis, herpesencephalitis. and parotiditic encephalitis all show an increasing tendency. 5. The Ch E in tuberculous meningitis and cerebral tumor show an increase in values, but in mental deficiency pigmentary xeroderma, and myasthenia the values stay normal.