Five multiparous Polish Red-and-White cows (in w k 8-12 of lactation) were assigned to a 5 x 5 Latin square design and fed total mixed ration TMR containing grass silage and concentrates representing 58 and 42% of dietary D M , respectively. The five treatments were dietary supplements of protected DL-methionine (Smartamine™ M ) fed in the following amounts: 0, 8, 16, 24 and 32 g/d. Dry matter intake yield milk, milk protein and fat, and contents of protein, fat and lactose in milk were not affected by the treatments. At the same time, NPN (%) was significantly affected (P<0.01), and protein-N and NPN (% of total-N) responded in an opposite manner (P<0.05). Of the plasma metabolites studied (β-hydroxybutyrate, glucose, and urea), only glucose levels were significantly (P<0.01) decreased by methionine supplementation. Plasma free methionine was significantly elevated (P<0.01) over the five treatments. In conclusion, the lack of apparent changes in the studied responses could have been due either to a higher methionine supply than that predicted or to a lower methionine requirement than that assumed. Consequently, a consistent relationship between increasing amounts of Smartamine™ M (0, 8, 16, 24, and 32 g/d) and milk protein responses of the cows, could not be described.