Changes in free amino acid concentrations is blood and various tissues were evaluated in cats adapted to the low-protein diet(20% protein, LPD) or the high-protein diet(60% protein, HPD) for 5 weeks. Cumulative body weigth gain for the 5 week period was 463$\pm$43g, and -128$\pm$40g for cats fed HPD and LPD, respectively. Feeding HPD significantly increased the size of liver and kidney. Cats adapted to HPD for 5 weeks have significantly elevated plasma concrntrations of essential amino acids (branched-chain amino acides, threonine, trytophan, phenylalanine and methoionine), whereas plasma levels of non-essential amino acids(alanine, asparagine, glycine, glutamine and serine) were significantly reduced in animals adapted to HPD(p<0.01, or p<0.001) compared to the values for the cats fed LPD. Changes in free amino acid concentratioks in whole blood induced by the variations in dietary level of protein closely reflect the pattern seen in plasma. Amino acids such as branched-chain amino acids, proline and threonine were most difficult to maintain homeostasis and consistantly elevated in lever, kidney, skeletal muscle and brain, as well as in blood of cats adapted to HPD(p<0.01 or p<0.001). All of the free amino acids in jejunum, excluding taurine and ornithine, were significantly elevated in animals adapted to HPD, most probably due to the rapid absorption of large amount of amino acids across the epithelium of small intestine.