Cerebral concussion was produced in rats by an iron pendulum hitting the external occipital protuberance. This resulted in loss of consciousness lasting from 3 to 10 minutes with prompt recovery and no focal neurological signs. The energy absorbed by the head at the impact was calculated to be about 1450 gm/cm. Light microscopic survey showed only minor pathological changes. However, electron microscopic observation revealed considerable alteration which began at 30 minutes, reached a peak at 1 hour, and disappeared at 24 hours after concussion. The salient changes included severe swelling of the neuronal mitochondria at the point of impact (occipital cortex), and extracellular edema at the site of contre coup (frontal lobe). Topographically, the most severe alteration was seen in structures at the craniospinal junction (medulla oblongata and upper cervical cord), consisting of both mitochondrial and edematous changes. Although there was no visible opening of the capillary interendothelial junctions, extravasated ferritin particles were accumulated in the edematous regions, indicating a transient increase in the permeability of the blood-brain barrier.