1. Myofibrillar protein breakdown was calculated from the urinary excretion ratio of Nτ-methylhistidine (3-methylhistidine) to creatinine in newborn premature and full-term infants. Representative values were obtained from single voidings provided that the infant's metabolic status was stable.
2. Nτ-Methylhistidine in infant urine was measured by a rapid Auto Analyser method and shown to give similar values to those obtained by ion-exchange separation techniques.
3. The molar excretion ratio of Nτ-methylhistidine to creatinine averaged 0·0159 in urine samples obtained within 12 h after birth. A similar ratio was found in amniotic fluid collected at birth. It is argued that this ratio does not reflect a low rate of myofibrillar protein breakdown in the foetus, but rather a more effective transplacental passage of Nτ-methylhistidine than of creatinine.
4. The urinary ratio increased during the first 2 days after birth to a plateau at 0·0372. This represents a myofibrillar protein degradation rate of 3·40% day−1 in full-term infants.
5. The molar excretion ratio during the period 40–120 h after birth increased in premature infants and reflects a fractional degradation rate of 5·34% day−1 in those infants weighing less than 1 kg at birth.
6. Lower excretion ratios were found in some infants of diabetic mothers and in athyroid infants.
7. The urinary excretion ratio of Nτ-methylhistidine to creatinine is presented as a useful method for evaluating the breakdown rate of myofibrillar protein in neonates and can be applied to a number of abnormal nutritional or hormonal states.