Rats were fed for 14 d on diets containing 50, 150 or 250 g/kg casein as the protein source. The daily excretion of Nt-methylhistidine (His(tau Me)), a non-re-utilized amino acid, was determined. 2. His(tau Me) excretion/100 g body-weight appeared to be unaffected by increasing the concentration of dietary protein from 150 to 250 g/kg. Assuming no change in the proportion of muscle in the animals these results are indicative of no change in myofibrillar protein catabolic rate. The excretion rate/100 g body-weight of the animal given 50 g/kg casein was lower than the other two treatments, especially towards the end of the 14 d treatment period. Thus at this time the myofibrillar protein catabolic rate was lower than in the animals fed on the higher protein diet. 3. In the animals fed on the high protein diet there was a tendency for this (tau Me) excretion rate/100 g body-weight to increase with age. 4. Nitrogen balance and creatinine excretion results are also presented.
