The aim of our present research is to evaluate the direct effects of humoral factors on protein metabolism. For this purpose we have adopted a method that enables measuring of parameters of protein and amino acid metabolism in isolated m. soleus and m. extensor digitorum longus of the rat. The rate of tyrosine release was used to estimate protein breakdown. The oxidation of leucine was evaluated by the rate of 14CO2 production from [1-14C]leucine. Incorporation of [1-14C]leucine into muscle proteins was used to estimate protein synthesis. The metabolic state of the muscles was evaluated by measuring changes in ATP, ADP, AMP levels, and calculated energy charge.