The radiactive nuclides were administered by gastric probe to rats with average activity 1.64, 1.48, 1.04, MBq/rat of ,59,Fe and 2.08, 2.12, 1.81 MBq/rat of ,60,Co respectively for young, adult and old groups. The cumulated internal doses to total body and R. M were calculated on the basis of radioactivity measurements on total body and organs. The results showed the chromosomal aberration yields of bone marrow cells and blood lymphocytes in experimental rats of various age-groups were higher than that of controls, the differences between the experimental and control groups were statistically significant at most dose points, moreover, the linear relationships existed between R. M cumulated doses and chromosome acentrics of bone marrow cells in old rats exposed to ,59,Fe and ,60,Co, the SCE incidences in lymphocytes of rats of various age-groups incorporated with ,59,Fe and ,60,Co did not greatly changed.