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I. Rats were chronically depleted of protein by being kept on a 6 % casein diet for 5-8 weeks. Control rats were fed on a normal diet. Both groups were injected intraperitoneally with ~-[U-'~C]lysine. Some rats from each group were then put on a protein-free diet to produce acute depletion. The animals were killed 3 days after the injection. 2. The organs and tissues were analysed for total nitrogen and radioactivity. Free lysine, total amino N and specific activity of free lysine were measured in muscle, liver and serum. The total muscle mass of the animals was determined. Samples of muscle and skin were fractionated and the sp. ac. of the fractions was measured. 3. The main loss of N in acute depletion was found in the viscera and carcass residue; the percentage of total body N contributed by muscle was increased in protein-depleted rats. 4. The depleted rats retained relatively more radioactivity in the internal organs and less in the carcass than normal rats. 5 . The ratio of the sp. ac. in protein-bound lysine to the sp. ac. of free lysine showed that protein synthesis was reduced in the muscle of the protein-depleted rats, although there was no decrease in the amount or sp. ac. of free lysine even in severe depletion. 6. Sarcoplasmic and fibrillar proteins of muscle were equally affected by protein depletion, but there was some indication of a preferential decrease in protein synthesis in one of the skin fractions. 7. The results for muscle protein are compared with those given in the literature for liver proteins. It is suggested that the rat adapts to a low-protein intake by an alteration in the pattern of protein synthesis, When a rat is given a single dose of a labelled amino acid the distribution of the label between the different organs and tissues of the body is dependent upon the dietary state of the animal. For example, 3 days after the injection of ~~-[~~S ] m e t h i o n i n e , in normal rats 75% of the radioactivity recovered was found in the carcass (muscle, skeleton and skin), 25% in the internal organs. I n rats on a low-protein diet these proportions were different; nearly 50% of the radioactivity recovered was in the viscera (Waterlow, 1959 In the experiments to be presented we have been concerned mainly with muscle, which, as the largest protein mass in the body, is of especial nutritional interest. We have tried to determine whether the reduced uptake of labelled amino acid by muscle in protein-depleted rats could be accounted for either by changes in the specific activity of the free amino acid, or by changes in the relative amounts and activities of the two main muscle fractions, sarcoplasmic and fibrillar proteins. Some observations have also been made on skin, which in the rat contains 25-30% of the total body nitrogen.In almost all these experiments the amino acid used was lysine. Most of the measurements were made 3 days after injection of the labelled amino acid, because it was found in previous work that this was the time interval at which differences in incorporation were most ...

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