1. Urine was collected in five healthy men over 10–14 days, with fasting blood samples on days 1, 5 and 10, whilst they consumed a standard creatine-free diet, which was quantitatively related to their body surface area.
2. The urinary excretion of 3-methylhistidine fell to a plateau by day 5 in all subjects. Myofibrillar protein catabolic rate calculated from the mean value of 3-methylhistidine excretion from day 5 to day 10 averaged 1·21 g day−1 kg−1 body weight. The average turnover of muscle myofibrillar protein was calculated to be 2·16%/day.
3. From a previous study using continuous intravenous infusion of l-[α-15N]lysine with serial muscle biopsies on the same subjects, the mean myofibrillar protein synthetic rate was calculated to be 0·82 g day−1 kg−1 body weight, and the mean turnover rate was 1·47%/day of total muscle myofibrillar protein.
4. The estimations of myofibrillar protein turnover rate derived from the two methods are compared and the differences discussed.