1. Urinary excretion of 3-methylhistidine, an index of the rate of muscle breakdown, has been measured during the first 7 days in patients after elective surgery or accidental injury. 2. There was no major difference between the mean daily excretion after skin grafting or total hip replacement, or in injured patients who were hyperketonaemic for the first 24 h after admission. 3. The group of injured patients who did not develop hyperketonaemia had a mean urinary 3-methylhistidine excretion which was twice that of the other groups. 4. It is concluded that increased breakdown of muscle protein makes a major contribution to the greater urinary nitrogen excretion in the normoketonaemic group of injured patients.

1. Venous blood concentrations of the branched-chain amino acids, valine, leucine and isoleucine, and urinary nitrogen excretion have been measured in sixteen adult males, from 2 h to 7 days after injury, and in four adults after elective skin grafts. 2. In the injured group the concentrations of these amino acids rose significantly 24 h after injury and had doubled at 4 days and remained high; in contrast the skin-graft patients showed no significant change. 3. In those injured patients with initial hyperketonaemia, defined as more than 0-2 mmo1/1, the increase in concentrations of branched-chain amino acids at the fourth and seventh days after injury was significantly less than in those with normoketonaemia, and was accompanied by lower urinary nitrogen excretion throughout the whole period. 4. It is suggested that the changes in the concentration of branched-chain amino acids after injury indicate decreased uptake by muscle or excessive release due to an imbalance between protein synthesis and protein catabolism in this tissue.

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