Adult male Sprague Dawley rats were subjected to spinal cord trauma of the lower thoracic-upper lumbar spinal cord using a weight dropping technique onto the surgically exposed dorsal surface. This experimental model, which results in severe paraplegia and development of complete segmental necrosis, was studied in a time sequence varying from immediately to 30 days afterward by electron microscopy, in order to characterize the pathologic changes in axons and myelin in the area of impounding. The results revealed tubulovesiculation within axons immediately posttrauma. Axonal fragmentation, axonal and adaxonal swelling accounted for the progressively increasing spongy appearance of the white matter. Beginning at 30 minutes and progressing until completion of tissue necrosis at 8 to 24 hours, axonal necrosis characterized by a finely granular alteration gradually evolved. Spheroids containing increased mitochondria, neurofilaments, lysosomes, and smooth endoplasmic reticulum appeared both in and peripheral to the zone of necrosis. Axonal calcification, rarely observed within 30 minutes, appeared maximally after the evolution of tissue necrosis. Two basic abnormalities of myelin were found: vesicular degeneration and intramyelinic vacuolization. These characteristically occurred late and involved fibers with abnormal axoplasm and were interpreted as being secondary in origin.