The life span of CBA mice was greatly reduced by exposure to 600 r, 1100 r, and 1300 r of X-rays given in 2 to 4 sessions 3 weeks apart. The resistance to further X-irradiations of these heavily irradiated animals was not reduced. The single acute lethal dose (i.e., LD50 (30 days) was unchanged so long as more than 3 weeks were allowed to intervene between the last sublethal dose and the test dose used to determine the LD50. The radiation does not hasten typical changes seen in unirradiated aging animals, such as alterations in collagen and the appearance of a liver lesion. The few facts so far available do not indicate that the process is similar to spontaneous aging. Cytotoxic chemical substances can also, under suitable conditions, cause a shortening of life span. Experiments are under way to examine if this effect is due to the production of somatic mutations or is due to destruction of cells. Preliminary indications are that the latter mechanism is more probable.