Hematological changes due to protein deprivation were studied in 34 young Japanese men who were given a standard diet with an N intake of 200mg/kg for one week and then low-protein diets with maintenance (45 +/- 2 kcal/kg) or excess (57 +/- 2 kcal/kg) energy and N intakes of about 44 to 99 mg/kg of whole egg or about 50 to 121 mg/kg of polished rice for three weeks. In the period of the low-protein diet the concentrations of most individual free plasma essential amino acids (EAA) decreased significantly in general, and thus the total EAA concentration decreased significantly. Lowering of the valine concentration and elevation of the alanine concentration were the highest changes. The total nonessential amino acid (NEAA) concentration increased significantly in men fed a low-rice-protein diet, but not in those fed a low-egg-protein diet. Consequently, in the former group the ratio of essential to nonessential amino acids fell significanly from 0.96 in the control period to 0.61--0.74 in the period of consuming the low-protein diet. The effect of protein deprivation on the plasma EAA concentration was also larger with egg protein than with rice protein, and the total EAA concentration of men fed egg protein changed significantly and in parallel with the N intake over the range of 44 to 99mg/kg. The decreases in serine and threonine and increase in alanine tended to be more when energy intake was over the maintenance level. The concentration of plasma proteins and especially albumin decreased significantly during the period of consuming the low-protein diet. The interrelation of plasma-free amino acids, the amino acid pool in tissures and dietary N and energy intakes is briefly discussed.