With the purpose of studying the mechanism of increase of urinary riboflavin excretion by Ca-gluconate, calciferol was administered to 7 rabbits. The behaviour of thiamine excreted in urine was just the same as that of riboflavin, i.e., the urinary excretion of thiamine was also inhibited. According to these results, vitamin D seems to control the urinary excretion of the vitamins which are activated by phosphorylation.