Plasma amino acid patterns were studied in sheep receiving varying amounts of DL-or L-methionine (0'6-10·0 g/day) or L-cystine (2·0 and 8·0 g/day) as abomasal supplements or DL-methionine (10 g/day) as a dietary supplement.Total amino acids in plasma decreased when small amounts (0'6-2'5 g/day) of methionine were infused, but increased substantially with the larger amounts of methionine (4'9-10·0 g/day). However, when methionine and taurine were excluded, the remaining amino acids decreased with all amounts of methionine. Cystine supplementation caused smaller changes in total amino acids.Methionine infusions significantly increased the concentration of methionine, cystine, taurine, and cystathionine in plasma. Cystine infusions increased the concention of the latter three compounds, but not that of methionine. Infusion of small amounts of methionine (0· 6-2 . 5 g/ day) produced only small increases in plasma methionine; with the infusion of larger amounts plasma methionine increased rapidly. The relationship between plasma methionine and amount of methionine infused was described by two straight lines which intersected at about 3 . 3 g/ day of methionine. With an infusion of 10 g/day the mean plasma methionine concentration was 235 ",moles/ 100 ml, which represented 64% of the total plasma amino acids. In contrast, an equimolar amount of cystine (8 g/day) produced only a small increase in plasma cystine, and had no effect on plasma methionine.Infusions of both methionine and cystine caused considerable reductions in the plasma concentrations of the branched-chain amino acids (valine, leucine, and isoleucine), and of serine and glycine.Dietary supplements of methionine (10 g/day) caused no change in the total concentration of amino acids in plasma nor in the concentration of any individual amino acid.