The author has already reported that urinary excretion of riboflavin increased by the administration of calcium salts to rabbits, while it decreased by calciferol. It was found in this experiment that when calcium salts were given to rabbits, the phosphatase activity of serum, liver, kidney, small intestine and muscle showed higher values compared with the control, but lower values when calciferol was administered with calcium salts. It may therefore be concluded that calcium salts and calciferol have an antagonistic action in regard to riboflavin excretion in urine.