Five multiparous Polish Red-and-White cows, weeks 8-12 of lactation, in a 5 x 5 Latin square treatment were fed a total mixed ration with grass silage and concentrates (58 and 42% DM). The five treatments were dietary supplements of protected L-lysine and DL-methionine (Smartamine™ ML) and DL-methionine (Smartamine™M) fed in the following amounts: I : 0 g/d + 0 g/d; II: 35 g/d + 0 g/d; III: 35 g/d + 10 g/d; IV: 35 g/d + 20 g/d; and V: 35 g/d + 30 g/d. The corresponding intestinal concentrations of lysine and methionine (% PDI) were: I: 6.89 and 1.88; II: 7.37 and 2.05; III: 7.32 and 2.29; IV: 7.31 and 2.54; V: 7.28 and 2.78, respectively. The D MI and milk yield were similar among treatments. In contrast, milk protein content was significantly increased (P<0.05) over the II treatment and then varied little. Milk fat content and yield varied inconsistently. Plasma metabolites such as glucose and (3-hydroxybutyrate were not affected and fell within physiological limits. Plasma free amino acids responded to the treatments, particularly lysine (PO.05) and methionine (P<0.001), producing a pattern of responses similar to that described for milk protein. Significant increases in milk protein content in the second treatment (Smartamine™ ML, 35 g/d) as resulting from improved postruminal supply of lysine (equal to needs), confirm the validity of the assumed requirement for this amino acid (i.e. 7.3% PDI). These increases could have also been due to a higher supply of limiting methionine. However, further improvements in intestinal supply of methionine, gradually meeting the assumed requirement (i.e. 2.5% PDI), had no effect on milk protein content. This lack of responses to methionine (treatments III-V), could have resulted either from a higher postruminal supply of this amino acid than that predicted or from a lower methionine requirement than that assumed.