Potencies of brain cells to DNA synthesis and proliferation were studied in two weeks old and adult mice in the norm and after the brain mechanical injury. No labeled large and middle neurons were found in the brain of intact and operated animals both under the pulse 3H-thymidine incorporation and saturation of mice with 3H-thymidine during 36 hrs. The same types of brains cells were labeled both in intact and operated two weeks old and adult mice: glial cells, cells of the subependymal zone, cells of the dentate gyrus inner margin, and sometimes, cells having characteristics of microneurons. The number of glial cells in the temporal cortex of intact mice diminished with the age. Under the brain trauma, the proliferative reaction of glia was expressed in a similiar way both in two weeks old and adult mice. The index of labeled cells in the subependymal zone is the same in these two age groups. With the age the cellular mass of subependymal zone decreases, rather than proliferative tendencies of supependymal zone. The brain traumatization resulted in the increase of labeled subependymal cell only under the direct injury of subependymal zone.