When 20mg/kg/day of Aureomycin was given to young rats, a promotion of growth was observed, while 200mg/kg/day of Aureomycin retarded the growth. In this case no parallelism was noticed between the nitrogen content of brain and that of liver tissue. The thiamine content and α-ketoglutarate oxidizing activity in the liver of rats, orally administered with 20mg/kg/day of Aureomycin, increased slightly in parallel at the biginning, but there was no considerable change on the whole, and the ratio of free to esterified forms of thiamine was not considerably changed. When Aureomycin was added to brain tissue homogenate of the control animals, α-ketoglutarate oxidizing activity was almost unchanged.