"The Chernobyl accident in April 1986 caused the deposition of radionuclides across Europe, followed by a long-term artificial increase of the radiation background [1]. In addition to the classical subject of mutagenesis after acute radiation exposure [2], the study of the time course of biological damage associated with chronic low-dose radiation exposure of mammals and the endeavors to predict biological damage in consecutive generations have become a relevant issue. Since there is little information on this topic [3, 4], the present work addresses these important questions. Starting with 1986, we were engaged in studying bio-logical effects of chronic low dose radiation in natural populations of bank vole (Clethrionomys glareolus, Schre-ber) in a series of many animal generations. The bank vole is a widespread rodent species that is used as indicator of environmental quality. It is a convenient object for many genetic tests, which originally have been devised for the laboratory mouse [5]. Comparison of own and literature data on doubling doses of acute irradiation for chromosome injuries had shown that the sensitivity of somatic cells of the bank vole to ionizing radiation is very similar to the sensitivity of human lymphocytes and germ cells of laboratory mice."