[en] Full text: The long-term course of the radionuclide activity concentration by natural populations of small mammals (bank vole and yellow necked mice) over 1986-1998 were established; the whole-body absorbed doses of external and internal irradiation with incorporated radionuclides were calculated; and time-course of whole-body absorbed dose rates and absorbed doses over 20-22 bank vole generations was determined. Genetic effects of low-dose chronic irradiation were proved in the range from close to background doses up to 10 cGy for germ and somatic cells of bank vole whose populations inhabited contaminated areas within 20-22 generations, as well as for germ and somatic cells of laboratory mice and pond carp. The recent general report on cancer incidence data in the cohort of atomic bomb survivors followed up by the Radiation Research Foundation give strong evidence that there is direct, statistically significant evidence of risk in the dose range of 0 - 0.10 Sv. Doubling doses were calculated by the chromosome aberrations test for the somatic cells of bank vole and showed that the doubling dose estimates for acute irradiation of somatic cells of bank vole and human lymphocytes of peripheral blood as well as for germ cells in laboratory mice are close to each other. Therefore the choice of bank vole as a model species for assessing genetic risk of low-dose chronic irradiation in human is justified. Transgeneration accumulation of genome instability was found by chromosome aberration tests and embryonal lethality in bank vole populations exposed to low-dose chronic irradiation within 20-22 generations (1986-1996). This transgeneration long-term effect of chronic low-dose exposure is detrimental because the genomes of animals in distant generations are more sensitive to the impact of very low radiation doses, as compared to the genomes of animals in a few first generations. Evidently, non-targeted effects of ionizing radiation such as genomic instability, bystander effects and other new phenomena have to contribute to short-term and long-term overall outcome after low dose of radiation exposure. We believe that the increased thyroid cancer incidence of children from irradiated parents would be the first manifestation of the induced genomic instability. Reverse relationship between genetic effects induced by ionizing radiation in somatic cells of bank vole and whole-body absorbed dose rates was proved. The doubling dose under chronic irradiation with a very low dose rate of 2-40 μGy/day is 2.6 cGy while under acute irradiation with dose rate of 5 cGy/min the doubling dose is equal to 31.4 cGy. The estimated excess relative risk (ERR) per Sv for the selected dose ranges of Life Span Study Cohort was the highest for the lowest dose category, namely from 0 to 20 mSv in the comparison with dose range of 0-2 Sv