1. Creatinine and Nτ-methylhistidine excretion rates have been measured in 13 hypopituitary children to calculate the body muscle contents and rates of myofibrillar protein breakdown. Analyses have been made during periods of growth hormone withdrawal and subsequent administration.

2. The creatinine excretion rate was lower in the hypopituitary children, indicating a lower muscle content per kg body weight. This difference persisted even in children who had received growth hormone for several years.

3. Excretion of Nτ-methylhistidine was reduced by the administration of growth hormone.

4. The fractional breakdown rate of myofibrillar protein, as calculated from the Nτ-methylhistidine to creatinine molar excretion ratio, averaged 1.76%/day in the four youngest children during growth hormone withdrawal. This was significantly higher than for control children of a similar age (P < 0.02) and was reduced to the normal rate of 1.47%/day by growth hormone administration.

5. in older children the fractional rate of myofibrillar protein degradation remained in the normal range irrespective of growth hormone treatment.

6. These results are discussed in the context of the anabolic effects of growth hormone on muscle being partly explained by its action to decrease rates of protein breakdown.

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