The effects of dietary protein and graded dietary lysine levels on plasma free amino acids (PFAA), nitrogen retention and egg production in laying hens were studied. There were no significant differences in the PFAA pattern and nitrogen retention between hens fed 15% and 12% dietary protein whereas the hen-day egg production was greater with hens fed the 15% protein diet. Supplementing the 12% protein diet with a nonessential amino acid mixture at the 2% level improved egg production to the level of hens fed the 15% protein diet. When hens were fed a diet containing varying amounts of lysine, plasma lysine remained at relatively low levels until dietary lysine was increased to a level where egg production and nitrogen retention were maximized but therefore increased rapidly. When dietary lysine was increased, plasma concentrations of essential amino acids except methionine and lysine, half cystine and tyrosine tended to decrease. The plasma taurine concentration decreased when dietary lysine increased from 0.35% to 0.55%. Thereafter, it was the same as at 0.55% lysine. The dietary lysine requirement of laying hens determined by plasma free lysine, nitrogen retention and egg production were 677, 664 and 687 mg/hen/day, respectively. The PFAA technique appeared to be useful in evaluating the dietary requirement of laying hens for lysine and perhaps other amino acids.