REDUCTION in metabolic rate has been shown to reduce radiosensitivity, but the effect of an elevated metabolic rate has not been well established. Similarly, anoxia induced by various means reduces radiosensitivity. In recent years, the reverse of this phenomenon has been applied in the attempt to increase the radiation response. This is based on the assumption that the radiosensitivity of tissue is related to the degree of oxygen tension in a given tissue at the time of irradiation1. Data on the protective action of myxœdema are not extensive; the hyperthyroid state has been shown to hasten mortality in mice given acute lethal doses of total-body irradiation2,3.