To study the relationship between fiber size and vulnerability to ischemia, we produced partial infarction of the proximal posterior tibial nerve of Wistar rats by injecting arachidonic acid into the ipsilateral femoral artery. In the resulting lesion degenerating fibers were concentrated in the center of the nerve. Distal to the infarct, surviving myelinated and unmyelinated fibers were evenly distributed throughout both fascicles. Smaller myelinated fibers (less than 6 micrometers in diameter) were more severely depleted than larger ones ( greater than 6 micrometer). Unmyelinated fibers were reduced in number in proportion to or to a greater extent than myelinated fibers. These findings demonstrate that large myelinated fibers are relatively less vulnerable to peripheral nerve ischemia than smaller fibers, and do not support the contention that unmyelinated fibers are inherently resistant to acute ischemia.