The relationship between post-traumatic hypophosphataemia, muscular high-energy phosphates (HEP) and red-cell 2,3-diphosphoglycerate (2,3-DPG) were studied in an experimental trauma model. Twenty-three anaesthetized pigs were submitted to trauma and observed for 72 hours. In group I (n = 10) and group II (n = 6) major trauma was inflicted with a high-energy missile. No phosphate supplement was given in group I, but group II pigs received 20 mmol phosphate/24 hours parenterally. Group III (n = 7) sustained only minor trauma, consisting of tracheotomy and catheterization of artery, vein and bladder, and received no phosphate in the post-trauma observation period. Reduction of serum phosphate was found in all groups after trauma. In skeletal muscle, decreased levels of HEP were observed after major trauma, whether or not phosphate supplement was given. Following minor trauma there was no alteration of muscle HEP. In red-cells, 2,3-DPG decreased slightly only after major trauma in pigs without phosphate supplement. Conclusions from the study were that hypophosphataemia was associated with cellular metabolic disturbance only after major trauma. In red-cells this disturbance seemed to be prevented by administration of phosphate. In skeletal muscle the phosphate supplement could not prevent fall in HEP levels, possibly because of a post-traumatic insulin resistance in muscle tissue, which may prevent uptake of phosphate into muscle cells.