Studies were made on alterations in plasma amino acids in young men fed a diet containing graded levels of wheat gluten. After one week on a standard diet containing 200 mgN/kg of mixed protein (animal protein content 45%), 38 young men were given a wheat gluten diet containing 170, 100, 60, 30, 15 or zero mgN/kg for 2 weeks. Blood samples measuring plasma free amino acids were taken before breakfast at the end of the periods on a standard diet and an experimental diet. In subjects on diets containing 170 to 30 mgN/kg the plasma concentrations of threonine, valine, methionine, leucine, tyrosine, phenylalanine, serine, histidine and arginine fell significantly with decrease in protein intake, but the concentration of alanine increased significantly. On the other hand, in subjects on diets containing 15 or zero mgN/kg, the plasma concentrations of the essential amino acids did not decrease, but increased to slightly more than in subjects on a diet containing 30 mgN/kg, and the alanine and glycine concentrations increased steadily. Values for plasma lysine varied from 146 +/- 22 to 194 +/- 31 mumoles/liter with gluten intakes of 170 to zero mgN/kg, but were comparable with that of 186 +/- 33 mumules/liter in subjects on a standard diet, showing that the plasma lysine concentration did not clearly reflect the dietary concentration of lysine in young men on a wheat gluten diet.