Secondary neurochemical events contribute to progressive tissue damage and subsequent neurological deficit after traumatic spinal-cord injury (SCI). Among proposed injury factors are alterations of phospholipids and certain cations. To clarify the relationship of membrane lipid changes (phospholipids, cholesterol, and arachidonic acid) to changes in tissue content of water and selected ions (sodium, potassium, and magnesium) after SCI, these variables were examined in spinal-cord segments from anesthetized ventilated rabbits subjected to laminectomy or to moderate (40 g-cm) or severe (150 g-cm) impact trauma at the lumbar (L2) segment. Trauma caused significant increases in tissue sodium, water, and arachidonic acid content, and significant decreases in phospholipids, cholesterol, potassium, and magnesium content. Alterations in magnesium were significantly related to injury severity. In contrast, changes in spinal-cord water content occurred to a similar degree in the two injury groups, as did tissue sodium and potassium content. Decreases in phospholipids were strongly correlated with decreases in tissue magnesium content, whereas changes in sodium and potassium were less well-correlated. Because magnesium ions play a critical role with regard to cellular bioenergetic state, calcium flux, amino acid receptor function, and eicosanoid production, reductions in tissue magnesium after injury may be important in the progression of secondary tissue damage.