Immobilization stress for 6 hours induced hemorrhagic erosion in the rat stomach. Hydroxyl radicals singnificantly increased in the pons-medulla oblongata in stressed rats. Mitochondrial superoxide dismutase (SOD) activity was enhanced in the midbrain but was lowered in the cortex, hippocampus and cerebellum in stressed rats. Thiobarbituric acid reactive substances increased by stress in the cortex and midbrain. These finding suggested that immobilization stress generated hydroxyl radicals and accelerated lipid peroxidation, and affected mitocho-drial SOD activity which may lead to neuronal damage in stressed rats.