An experiment was carried out to find out a good index which will give the best information on the protein intake and nutritional status. Three healthy adult men were employed as subjects of this experiment, and they were given the following experimental diets: 1st period, normal diet for the Japanese adult men, 2nd period, low protein, low protein value diet, and 3rd period, high protein, high protein value diet. During each period amino acid levels in plasma and in urine were estimated, and the following results were obtained. 1) Amino acid concentration in blood did not reflect the intake, only tryptophan showing the tendency to be a little low in the low-protein period. 2) The excretion of tryptophan when expressed per kg. body weight was correlated with the intake, but the other amino acids did not always agree with the different dietary levels. 3) The excretion ratio of the amino acids, however, reflected the state of the intake most sharply, and all four amino acids, lysine, threonine, methionine and tryptophan, were of the highest excretion ratios in the low-protein period.