The experiment was performed to observe an influence of excessive dose of thiamine derivatives upon the hepatic CoA concentration in young rat. The animals used in this experiment were young male rats weighing 50g, and were divided into control (A) and thiamine derivative administered (B) groups. Group A was given orally 18% casein diet and group B was given 18% casein diet with 500μg of S-benzoylthiamine O-monophosphate per rat daily for 3 weeks. Hepatic CoA concentration was determined by Kaplan・Lipmann method at 1st, 2nd and 3rd week of feeding. As a result, the hepatic CoA concentrations of group B were 85 to 100 units/g and reduced significantly as compared to 120 to 150 units/g in group A. The hepatic CoA concentration revealed the same tendency when thiamine tetra-hydrofurfuryldisulfide was administered as an other thiamine derivative.