The introduction of somatic mutations, represented by chromosome aberrations, as a result of radiation in young mice does not immediately produce the conditions of age. Rather they are part of an interacting system whose expression is checked in more youthful and vigorous stages by countering factors. In the non-pathological condition, some cells are lost as the prolonged result of deleterious somatic mutations present in the nuclei. The morphological integrity of chromosomes during several months of chronic gamma radiation is maintained. Severely damaged cells that have been exposed to fast neutrons can recover their ability to divide. The somatic mutation theory evidenced by the occurrence of chromosome aberrations with age is applicable in liver cells of cortrol mice, and is regarded as a measure of the inherited rate of aging. (auth)