In rats orally administered with 200 mg/kg/day of Aureomycin, the thiamine content and α-ketoglutarate oxidizing activity in the liver decreased for the first two weeks, increased slightly in the third week, and again decreased in the fourth to sixth week, always both in parallel. In the brain both did not change till the forth week, they, however, decreased in the fifth to sixth week. Ratio of free to esterified forms of thiamine did not decrease in brain and liver. When cocarboxylase was added to homogenized brain or liver tissues with low thiamine content and α-ketoglutarate oxidation activity, an imperfect improvement was observed. While in the control animals no influence was caused. The magnecium ion content which is indispensable for α-ketoflutarate oxidation was 9×10^<-3> M in brain and 6×10^<-3> M in liver. In addition of 4μg/ml of Aureomycin or Achromycin, in vitro, α-ketoglutarate oxidation was checked by chelation to Mg". This action was stronger in brain than in liver.