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.