In order to elucidate the mechanism of the high blood contents of vitamin B_<12> and increased urinary excretion, of B_<12> in human leukemia, the study of B_<12> metabolism in rats following whole body ^<60>Co γ-irradiation was undertaken. It is a well known fact that peripheral leukocytes show a considerable decrease within 3 days following LD_<50> irradiation, but when B_<12> metabolism was examined in the rat during the same period, it was found that the B_<12> level in blood showed a temporary significant increase during the 3 to 24 hour period following irradiation. Urinary excretion of B_<12> in the rat, even in controls, is more than 30 times as that in the human body, the weight of the rat being converted to that of the human body, but when the rat was irradiated, a significant increase in urinary excretion of B_<12> was found within 48 hours. It was postulated by the author that the increase in B_<12> level in blood within 24 hours, and increase in excretion of B_<12> within 48 hours after irradiation are attributed to B_<12> release from leukocyte destruction.