After ablactation, wistar strain white male rats, weighing 270g and 340g, were fed with a diet of CLEA for three months. The whole daily excretion of each feces and urine were collected, and extracted with water($80^{circ}C$ hot water). The combined extraction were filtered and the $B_2$ was determined with the parts of the filterates by the lumiflavin fluorometric method, and the FMN, FAD and FR with the rest of the filterates by paper chromatography. The following results were obtained; 1. $B_2$ contents in the feces were $27.52{gamma}$ per 100 grams per body weight, and $83.93{gamma}$ per each rat per day. 2. $B_2$ contents in the urine were $18.47{gamma}$ per 100 grams per body weight, and $56.33{gamma}$ per each rat per day. The total daily excretion of $B_2$ contents in the feces were 1. 5 times as much as in the urine. 3. Among the total daily $B_2$ excretion of one white wistar strain rat in the feces were the following ; FAD, 81.0% ; FMN, 14.9% ; FR, 3.3%. Therefore the order of the contents were FAD>FMN>FR.