In the previous paper (2), the effect of inosinic acid on the activation of thiamine diphosphate (TDP) in liver-pyruvate oxidase system was studied, and the increase in the activity of the enzyme system was confirmed by addition of inosinic acid (IMP) or adenosine triphosphate (ATP) in vitro.Further, feeding experiments were repeated with thiamine-deficient rats, and the rise in activity of liver-pyruvate oxidase system was again recognized by oral administration of thiamine and IMP.Further investigation was made to elucidate whether other purine morionucleo tides activate also the enzyme system.Adenosine monophosphate (AMP) and guanosine monophsophate (GMP) showed also the marked increase of the enzyme activity (1).In the parallel experiments, the fate of the nucleotides in the reaction mixture were carefully tested at the end of the experiment by paper chromatography.It was always found that there were several pathways for the degradation of the mononucleoti des (3).The catabolism of inosinic acid and inosine used here were then studied in vitro using rat-diver homogenate.The preliminary test was performed for the stability of the nucleotide and nucleoside by treating with trichloroacetic acid, and the metabolic degradation products were studied by paper chromatography. The results sernied to suggest that inosine might accelerate the activation of thiamine diphosphate in the liver-pyruvate oxidase system (1, 3).Therefore, final attempts were made for confirmation whether inosine or hypo xanthine plus D-ribose activates the above enzyme system. The addition of inosine or hypoxanthine plus D-ribose was found to accelerate the liver-pyruvate oxidase system, whereas no activation was seen by the single addition of hypoxanthine. At the end of the experiment with hypoxanthine and D-ribose, the reaction mixture was tested by paper chromatography, and the reappearance of inosine was always noticed (1).Finally, from these findings, the present authors have been led to the conclu sion that the metabolites produced by i nterconversion of nucleosides and the de gradation products may contribute to the activation of thiamine diphosphate in liver-pyruvate oxidase system.