Cyclo-oxygenase inhibitors and free-radical scavengers protect the skin against necrosis induced by frostbite. However, the tissue component(s) that determine the evolution of skin necrosis and the mechanism of this pharmacologic protection are not precisely defined. We have studied freezing injury to rabbit ears by serial biopsies examined by light and electron microscopy. The morphologic evidence of skin injury due to freezing was localized exclusively in the endothelial cells, particularly in the arterioles. Within 1 hour, the entire microvasculature demonstrated endothelial damage. Intravascular platelet aggregation occurred just after thawing and closely paralleled the endothelial cell injury. Very few neutrophils were seen initially (at 10 minutes). By 1 hour, leukocyte aggregates were present, and they further increased at 6 hours. Swelling of the interstitium started 10 minutes after thawing, while extravasation of erythrocytes began to appear by 6 hours. Parenchymal elements of skin were relatively free of damage. In the ear cartilage, the chondrocytes showed evidence of damage immediately after freezing. The administration of superoxide dismutase (SOD) during thawing (reperfusion) did not qualitatively alter any of the initial morphologic changes induced by freezing. We conclude that the endothelial cell is the initial target of injury induced by freezing, an initial injury that is mediated by a non-free-radical-mediated mechanism. It is likely that this acute injury ultimately compromises blood flow and leads to skin necrosis.