1. Metabolic activity of inosine on the incorporation of riboflavin into rat-liver xanthine oxidase system was studied.
2. The effect of ingested inosine on liver nitrogen was scarcely recognized.
3. The tendency of the increase in total riboflavin and flavin mononucleotide in the liver of the rats receiving inosine was observed, but the change in the contents of free riboflavin and flavin-adenine dinucleotide was not observed.
4. The tendency of the increase in the activity of liver xanthine oxidase in the rats receiving inosine was observed.