Dose response and dose-rate effects in the low-dose-rate (LDR) range have not been evaluated with respect to chromosome aberrations. The incidences of chromosome aberrations were analyzed in splenic lymphocytes from mice that were continuously exposed to γ-ray at the LDR for evaluation of the dose response and dose-rate effects. The frequency of dicentric chromosomes and translocations detected by fluorescence in situ hybridization (FISH) increased almost linearly up to 8,000 mGy at 0.91 mGy/h and to 700 mGy at 0.045 mGy/h. In no-exposed mice, translocations increased in a linear quadratic manner with age. After adjustment for age, value of linear coefficient of the dose-response relationship for the frequency of translocations at each dose rate decreased as the dose rate was reduced, suggesting a dose-rate effects in the LDR range. These results will be useful for establishment of a biodosimetry method for individuals who are occupationally or accidentally exposed to chronic LDR radiation. On the basis on the present findings, retrospective dosimetry was performed for two populations of subjects who had been exposed to radioactive fallout from nuclear explosion tests. The merit and drawbacks of using dicentric chromosomes or translocations as indices in chronic exposure at the LDR are discussed.