An administration of p-phenylenediamine on albino rats caused a distinct decrease in the total riboflavin contents of various organs. In the case of weekly administration of 1/1000 amount of fatal dose, total riboflavin and FAD decreased in liver, while in the stomach free riboflavin increased, and the total amount, FAD and FMN decreased. In the case of daily administration of 1/500 amount of fatal dose, total riboflavin and FAD decreased in liver and in the kidney FAD decreased and FMN increased.