Twelve healthy adult men (aged 20-23) were administered a multivitamin preparation containing 13 or 26 μg of vitamin B_<12> (cyanocobalamin) every day for 7 days, and examined for changes in the plasma concentration and urinary excretion of vitamin B_<12>. 1) The daily, oral administration of 13 μg of vitamin B_<12> for 7 days caused practically no changes in the plasma vitamin B_<12> concentration. 2) The daily, oral administration of 26 μg of vitamin B_<12> for 7 days caused a tendency to slight elevation of plasma vitamin B_<12> concentration throughout the administration period. 3) Vitamin B_<12> was not detected in the urine of any subject before the administration of the multivitamin preparation but definitely detected in the urine of 2 of the 4 subjects administered 26 μg/day of the vitamin on days 5-7 of administration.