This thesis is primarily concerned with the adaptation of leucine oxidation in rats that are either fasting or have been fed protein restricted diets. It also includes the adaptation of valine oxidation in rats fed a protein-free diet for periods of up to 3 weeks. The initial experiments were concerned with resolving the differences of opinion which held in contention the ability of the malnourished rat to adapt to a low-protein diet and in particular to conserve the branched-chain amino acids. Much of the disagreement was later found to be due to the choice of the radio-active label for measurement of output following a tracer dose of the labelled for measurement of I4/CO2 output following a tracer dose of the labelled branched-chain amino acid (B.C.A). Parallel studies on the first two enzymes concerned with the catabolism of the B.C.As were also carried out in liver and gastrocnemius muscle to understand better the control mechanism and chief site of oxidation. The preliminary studies demonstrated dehydrogenase activity in skeletal muscle a a well as in liver. It was the first demonstration of this enzyme activity in rat skeletal muscle. Moreover, the degree of enzyme adaptation led to the hypothesis that skeletal muscle was the chief site of both oxidation and its control. Further studies were carried out to observe the effects of feeding a protein-free diet to rats of varying ages for periods of 1, 2 or 3 weeks. Enzyme activities and oxidation were measured. This gave further support to the original hypothesis. Subsequently a more accurate measure of skeletal leucine oxidation was developed. By using the method of a constant tall rein infusion of a radioactive tracer in vivo it was possible to estimate accurately the total body flux of leucine, together with total body protein turnover and rates of leucine oxidation. Similarly, by giving a constant infusion of the same radioactive tracer to the perfused hind-limb of an identically treated rat, it was possible to arrive at a more accurate measure of the contribution of skeletal muscle to total body leucine oxidation.