Decreased riboflavin concentrations were again found in different tissues from old rats (23 months of age) as compared to younger animals (11 months of age). This decrease was observed in the liver, heart and brain (-7 to -10%) and mainly in the kidneys (-60%). In the first three organs it is due to a decrease in flavin adenine dinucléotide (FAD), whereas most of the time the flavin mononucleotide and free riboflavin (FMN + FR) contents remained unchanged. In the kidneys, both FAD and FMN + FR are decreased, the latter more so than the former. This phenomenon could be explained by a modification in the enzymatic activities involved in the riboflavin FMN-FAD metabolic pathways and would result from a decrease of thyroid activity during aging. In the spleen, the total riboflavin concentration is increased in old rats without changes in the relative ratios of FAD and FMN + FR. No particular effect was observed in lung and muscle tissue during aging