1. Myofibrillar protein degradation has been measured by the rate of 3-methylhistidine excretion in premature infants weighing between 635 g and 1295 g. Analyses were made in conjunction with 1–3 day nitrogen balance studies.
2. In 56 balance studies in 36 infants, total muscle protein breakdown varied between 0·70 and 2·58 (mean 1·05) g day−1 kg−1 body weight while the percentage of total muscle protein degraded each day was between 3·3 and 8·3 (mean 4·8).
3. Both total and fractional rates of protein breakdown showed highly significant negative correlations with nitrogen retention but no relationship to total energy input.
4. Protein degradation was higher than average in infants who were losing weight at the time of the balance study, lower in infants who were gaining weight and higher in those who died within 2 weeks of the analysis.
5. Myofibrillar protein breakdown was not different between infants fed orally and those receiving total parenteral nutrition.
6. Generally the effects of nitrogen and energy status on muscle protein degradation in the premature infants are different from changes reported in adult human beings or adult rats. We suggest that this difference may be a consequence of the very limited energy reserves of the premature infant.