Ammonia of the rat brain in case of the injection of chlorpromazine was measured by Conway's microdiffusion method, and amino acid by the paperchromatography, and the following results have been obtained in the respective measurements. 1) Following the injection of chlorpromazine (50 mg/kg per once), the ammonia contents of the rat brain decreased to 0.34 mg. per cent. 2) In case of repeated injection of chlorpromazine (20 mg/kg every day), the ammonia contents of rat brain increased to 0.93 mg per cent, and the simultaneous decrease of glutamic acid was not found. 3) The amounts of ammonia formed by above mentioned brain slices were found to be 6.2 μM/g. for the first one hour, 7.1 μM/g in two hours, 9.3 μM/g. in three hours and 12.7 μM/g in four hours, respectively. Namely, in this case, the increase of ammonia formation was found during the first one hour and the decrease of it was found thereafter.