Dynamics of neuroepithelial cells reaction was studied in different areas of brain obtained from mouse embryos affected by X-rays irradiation on 10th, 12th and 13th days of pregnancy. It was shown that as differentiation starts after the irradiation regeneration processes arise along with cell death. This develops in all brain areas due to total mitotic division of unchanged neuroepithelial cells. Forming of rosettes on proliferative centers is mainly characteristic for the forebrain. After the irradiation of mice at 1-2 days pregnancy rosettes form only in rostral part of anterior encephalic vesicle. In irradiation on 13th pregnancy day a significant number forms in rostral, lateral and dorsal parts of the embryonic forebrain. Irradiation on the 14th day causes the forming of the rosettes in the forebrain ventral part.