The sciatic nerves of 12 male rats were examined in the electron microscope 14 days after pneumatic tourniquet compression. Tourniquet pressure was maintained at 300 mmHg for varied lengths of time (30 minutes to 3 hours). Nerves compressed for 30 minutes showed very mild fissuring of the myelin without axonal degeneration. Examination of nerves compressed for 1 to 3 hours showed progressively more varied and extensive damage. Changes included splaying of myelin lamellae, axonal shrinkage with periaxonal edema. Schwann cell hypertrophy, and an increase in the number of microtubules and mitochondria per unit area. The myelin sheaths of some fibers, compressed for more than 2 hours, were completely ruptured. These changes resemble nerve lesions which could be induced by a variety of experimental procedures. Ultrastructural changes produced by tourniquet compression are apparently time-related and affect large-diameter nerves more profoundly than smaller-diameter nerves. The data reported provide an explanation for delayed muscle rehabilitation experienced by patients who have undergone extremity surgery with pneumatic tourniquet application. The evidence presented suggests that the incidence of tourniquet palsy may be far greater than previously recognized.