This study was undertaken to investigate the effects of ruminally protected amino acids (Methionine and Lysine) on in vitro ruminal parameters, and in vivo milk yield and milk composition in mid-lactating cows. In the first in vitro experiment, there were no statistical significances between treatments in ruminal pH and dry matter digestibility during various incubation times. In the second in vivo experiment, milk yield decreased by 11.92% in control and 5.68% in the treatment respectively,

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... decrease rate of milk yield in the treatment was lower than control. Milk yields naturally decreased as time goes by since the DIMs (Days in milk) of the cows in experiment were in mid-lactation period. 4% FCM (Fat corrected milk) and milk protein yields also, respectively, decreased by 11.25% and 11.09% in control and 6.16% and 5.47% in the treatment as compared with the intial. Milk protein and milk fat production were higher in the treatment (0.90 kg, 1.10 kg) than those of control (0.66 kg, 0.79 kg). Milk fat content significantly increased with supplementing protected amino acids as compared to control (P<0.05). From the above results, protected amino acids were positively utilized in the performances of mid-lactating cows without inhibiting rumen fermentation. Further investigation is suggested for essential amino acid composition and intestinal digestion rate out of rumen bypass protein in dietary protein to be estimated. (