Following contusion (500 g-cm) at upper thoracic levels, sections from the spinal cords of 13 rhesus monkeys were examined with the electron microscope. Survival times ranged from 4 hr to 10 weeks. Samples were taken from the lesion site, from areas 3 and 10 mm rostral and caudal to the lesion center, and from the lumbosacral cord. Four hours postoperatively, several small axons located close to the grey matter at the lesion site exhibit abnormal accumulations of organelles including mitochondria, dense bodies, vesicular structures, and multivesicular bodies. By 12 hr postoperatively many axons at the lesion site appear to be swollen with organelles and exhibit thinning of their myelin sheath. Some organelle-rich profiles lack a myelin sheath altogether. At this time dark axons are present, and myelin sheaths which appear to be empty or to contain small amounts of flocculent material. By 18 hr the first signs of axonal changes appear in the tissue taken 3 mm from the center of the lesion, both swollen and pyknotic axons being present. The axonal pathology spreads from the central part of the cord to the periphery at the impact site, and from the impact site rostrally and caudally, beginning at 18 hr and continuing for the duration of the study. Small fibers degenerate first and large fibers later. The axonal changes observed appear to be comparable to those reported for the central and peripheral nervous systems in other species.