✓ The ultrastructure of the microvasculature of the white matter of the spinal cord was studied after experimentally induced trauma in the cat. Immediately after the induction of trauma, disruption of endothelial cell junctions, increased pinocytotic activity in endothelial cells, and perivascular edema were seen in the blood vessels at the site of injury, but not in those of adjacent segments. However, within 2 hours of injury, the blood vessels of the white matter of the rostral and caudal segments also showed evidence of increased endothelial cell pinocytotic activity and perivascular edema, but disruption of endothelial junctions was not seen. It is therefore concluded that vasogenic edema occurs in the white matter at the site of injury soon after trauma and is due both to leakage from vessels with damaged endothelial cell junctions and also to increased transvesicular transport. By contrast, vasogenic edema develops only after a lapse of time in segments rostral and caudal to the site of injury, and probably results from increased transvesicular transport. A possible role for neurogenic chemical mediators in the genesis of the perivascular edema is discussed.