Experiments on mice were made to examine mexidol effects on the course and outcome of acute radiation sickness (ARS). Regenerative processes in hemopoietic system were studied after 6.0 Gy radiation, survival of the animals was studied in 7.7 Gy radiation. Mexidol was used in two schemes. Scheme one--once a day on days 1-8 after radiation. Scheme 2--twice a day on days 1-5, and once a day on days 6-8 after radiation. Administration of mexidol after exposure to ionizing radiation promotes activation of postradiation recovery of erythropoiesis, neutrophilopoiesis in bone marrow and lymphopoiesis in the thymus. This effect was stronger in mexidol administration twice a day. After ionizing radiation in a dose 7.7 Gy only 56% mice survived. Twice a day mexidol increased the survival to 94%. Once a day mexidol had insignificant effect on survival of mice. Thus, administration of mexidol on days 1-8 after radiation had a good effect on metabolism, stimulates regenerative processes in hemopoietic system, attenuates severity of ARS and increases survival of irradiated animals.