Immobilization stress in rats can provoke damages of the brain parenchymal vessels, which are most pronounced in the reticular formation of the midbrain. In this case the blood plasma and cells enter the brain and the blood elements of brain tissue enter the circulation. Some experimental animals exhibit the serum complement-fixing brain antibodies and specific reactions of basophils to brain antigens 14 days after exposure to stress. Most of the rats reveal immune reactions and neurosensitivity a month later. Some of them exhibit the autoantibodies to norepinephrine and serotonin. It is suggested that stress-induced brain vascular damages may play an important role in the mechanisms of immune reaction induction.