In the model of severe traumatic brain injury (TBI) in adult and old Wistar rats we have demonstrated significant changes in the structure of sensorimotor cortex and hippocampus, resulting in a significant decrease in the number of neurons, growth of vacuolated hyperchromic nerve cells fraction and the sharp increase in the number of glia. Gene therapy by means of cationic liposome-mediated APOE3 transfer significantly decreased reactive gliosis and number of microgliocytes both in the cortex and in the hippocampus, and had a positive effect on the number of neurons, preventing their loss in adult and especially in old rats.