Brain damage in the early stage of experimental head injury was studied under electron microscope, especially with respect to the permeability of the blood brain barrier and damage to nerve fibers. Lightly anesthetized rabbits were treated by intravenous injection of horseradish peroxidase (150-250 mg/kg) and were subjected to occipital impacts with a pendulum impactor. The animals were then perfused with a 2.5% glutaraldehyde mixture. Small blocks of the brain were sampled, incubated, postfixed, and embedded in Epon®. These sections were observed under an electron microscope without staining. The authors' criteria of concussion have been previously published. There were three nonconcussion, 11 nonlethal concussion and seven lethal concussion animals. All cases of lethal concussion showed subarachnoid hemorrhage (SAH) in the brainstem and 57% of them showed microscopic parenchymal perivascular hemorrhages in the brainstem. Fifty-five percent of the nonlethal concussion cases showed SAH in the brainstem with a little parenchymal hemorrhage. The nonconcussion cases showed no hemorrhagic change except contusions and/or SAH of the cerebellum. Hemorrhagic changes were frequently seen in the cerebrum and cerebellum of nonlethal and lethal concussion cases without correlation to the severity of concussion. Under electron microscope, nonconcussion cases showed mild splitting of myelins. In nonlethal concussion cases, swelling of perivascular astrocytic feet was seen to a moderate degree in the cerebrum, pons, and medulla oblongata and damage to axons and myelins were mainly seen in the pons and medulla oblongata. Many cored dense bodies containing peroxidase were diffusely recognized in vascular endothelial cells. In lethal concussion cases, beside findings seen in nonlethal concussion, intravascular peroxidase was seen to leak from the intravascular space through an opened tight junction to the extracellular space, axons and neuropils. Severe splitting of myelins and atrophic axonal changes were seen in the pons and medulla oblongata. In conclusion, irreversible damage to the blood brain barrier and nerve fibers was seen in the brainstem of lethal concussion cases. Reversible changes of increased permeability of the blood brain barrier and mild damage to nerve fibers were seen in nonlethal concussion cases. Even in nonconcussion cases, some minimal damage to myelins was seen.