To assess the viability of traumatized skeletal muscle, only clinical criteria have been available. Discolouration of the muscle following the trauma is one such criterion, and was earlier found to be associated with impaired circulation and severe metabolic disorders. The present study, continuing investigations into the pathophysiologic basis for the clinical criteria, was concerned with local electrolyte and water changes in skeletal muscle subjected to high-energy trauma. Biopsies were made from discoloured traumatized muscle and from adjacent normally coloured muscle 1, 5 and 10 hours after the trauma. Discoloured muscle showed little change in water content, but elevated values of Na+ and Cl- and very low K+ and Mg++ values. The changes progressed with time and showed no tendency to reversibility. The adjacent normally coloured muscle did not differ in electrolyte or water content from nontraumatized muscle. The magnitude of the electrolyte changes in discoloured muscle implied very severe cellular damage. This could have been due to mechanical disruption caused by the pressure wave, or to inability of the muscle cell to maintain a normal transmembrane potential, or to both factors. The findings correlate well with results of earlier metabolic and circulatory studies indicating that discolouration of traumatized skeletal muscle signifies nonviability.