Twelve multiparous Polish Red-and-White cows, were assigned to two-factorial (2x2) arrangement of treatments, in a balanced changeover design, involving two levels of dietary protein (PDI: 100 vs 85% of the INRA requirement) and two levels of ruminally-protected DL-methionine (Smartamine(TM)M: 0 vs 20 g/d). The cows were in wk 8-12 of lactation and were fed a mixed diet with grass silage and concentrates representing 50 and 50% of the dietary DM, respectively. The four combinations of treatments were: 100% PDI (0 g Met), 100% PDI (+20 g Met), 85% PDI (0 g Met) and 85% PDI (+20 g Met). The calculated intestinal concentrations of lysine and methionine (% PDI) were: 6.91 and 1.79, 6.88 and 2.27, 6.88 and 1.87, 6.84 and 2.42, respectively. Dry matter and net energy (UFL) intakes were similar across the treatments. Crude protein, PDIN and PDIE intakes were significantly (P<0.001) higher in cows fed the 100% PDI diets, as compared to those fed the 85% PDI diets. Milk and 4% FCM yields and the efficiency of milk production were not significantly different. Milk fat, lactose, and SNF contents did not respond to the treatments. Feeding the 100% PDI diets compared to the 85% PDI ones increased significantly (P<0.05) milk protein yield (from 0.67 to 0.76 kg/d). Milk NPN and urea concentrations were significantly (P<0.001) decreased, and the relative concentration of milk protein N (% of total N) was significantly (P<0.05) increased in milk from cows fed the 85% PDI diets compared to the 100% PDI diets. No effects of methionine supplementation and no interactions between dietary protein intakes and supplemental methionine levels on the variables addressed in this study were detected. It is concluded that supplementing ruminally-protected methionine to grass silage-based diets did not result in any apparent changes in lactational responses, regardless of the protein status of these cows, thus indicating that methionine is not the first limiting amino acid in these diets.