Ammonia of the normal rat brain was measured by Conway's microdiffusion method, and amino acid by the paper chromatography, and the following data have been obtained in respective measurements. 1) In the case of the normal rat brain immersed in liquid nitrogen, the percentage of the contents of the following substances proved to be as follows: ammonia, 1.05 mg. per cent; glutamic acid, 140.9 mg. per cent; glutamine, 92.3 mg. per cent; and γ aminobutylic acid, 27.7 mg. per cent. 2) Trepanation and Enuculation of the Brain. Contents of the substances found in the brain frozen solid by dry ice acetone gave the following percentage; ammonia, 0.64 mg. per cent; glutamic acid, 159.9 mg. per cent; glutamine, 94.6 mg. per cent; and γ aminobutylic acid, 37.2 mg. per cent. 3) The amounts of ammonia formed by brain slices were found to be 5.1 μ M/g. for the first one hour, and 7.3 μ M/g. in two hours; and 13.8 μ M/g. in three hours, respectively. In this case, the decrease in the contents of glutamic acid and glutamine during the first two hours paralleled the increase in the contents of ammonia.