There are many reports on the protein metabolism of the brain but it seems that much remains to be clarified as regards the structural chemistry of the brain proteins. For the purpose to elucidate this problem, the author studied histological changes in the cat brain at the time when the frontal lobe was placed under chronic compression by the extradural compression method of Ishii et al., and simultaneously studied the incorporation of (14)C into each subcellular unit protein of the brain tissue after intravenous infusion of [U-(14)C]-glucose. Further, the incorporation of (14)C into what Otsuki et al. call acid-ethanol soluble proteins and residual proteins was investigated. Brain proteins were fractionated by DEAE Sephadex column chromatography as well as by the disc-electrophoresis with polyacrylamide gel, and compared these fractions with these of normal cat brain. The results of the study are briefly summarized as follows. High molecular substances of the mitochondrial fractions of the cat brain under the chronic compression showed a higher relative specific activity (RSA) to the blood glucose than those of the normal cat brain. In the case of acid-ethanol soluble proteins in the brain under chronic compression, just as in the case of normal cat brain, particulate fractions, especially crude mitochondrial fractions, revealed a higher incorporation of (14)C, and also their RSA was higher than that in normal brain. In the fractionation by DEAE-Sephadex column chromatography, the mitochondrial fractions of the brain under chronic compression showed a specific peak of the eluate that dissolved with 0.8 M NaCl. This substance was the protein fraction that showed the mobility identical with prealbumin fraction obtained by the dis-electrophoresis with polyacrylamide gel. Looking at the (14)C incorporation by each protein fraction separated on DEAE-Sephadex column, the radioactivity was observable in the portion dissolved only by 0.02 M sodium phosphate buffer. This substance, when subjected to the disc-electrophoresis with polyacrylamide gel, showed the mobility identical with γ-globulin fraction.