The method employed consists of three stages: (1) The conversion of the total nitrogen into ammonia by the Kjeldahl process; (2) the conversion of the ammonia into pure aqueous solution; and (3) the calorimetric estimation of the ammonia in this solution by means of Nessler's solution. The result is expressed as “nitrogen number,” which denotes the number of hundredths of a milligramme in 1 c.c. of cerebro-spinal fluid. In general paralysis there is a high nitrogen number, which is marked towards the termination of the disease. It may be low in the early stages or in remissions. In mania the nitrogen number is always low—a factor of diagnostic importance in distinguishing between this disease and maniacal phases occurring in general paralysis. In imbecility, dementia præcox, paranoia, amentia and epilepsy the nitrogen number varies, but is usually low, and in terminal dementias it is very high. It was also found that density and nitrogen numbers run parallel to one another, with the exception of epilepsy, where high density obtains. The author emphasises the fact that choline does not occur in the cerebro-spinal fluid in mental diseases, and that ammonium salts only occur in the merest traces.