Two experiments were conducted to investigate the effects of free Met and Lys supplementation on ruminal fermentation in vitro and in vivo. In the in vitro study, supplementation of Met and Lys had effects on concentrations of total VFA and NH4+, proportions of acetate, propionate, butyrate, isovalerate, valerate, and the ratio of acetate to propionate. Percentages of microbial N synthesis in effluent pellets were not affected by supplementing Lys and Met. The combination of 0.52% Met and 1.03% (90% DM) Lys resulted in the highest concentration of total VFA, second highest concentration of NH4+, and lowest ratio of acetate to propionate with no decrease in the microbial CP production. In the in vivo study, supplementing 0.29% Met and 2.27% Lys (100% DM) had no impact on mean DMI, OMI, milk yield, milk component production or percentage, SCCS, 4% FCM and ECM production efficiencies, or body weight gain. Although DMI for the treatment group was numerically lower than the control group, milk yield, % milk fat, and % milk protein for the treatment group were numerically higher than the control group and resulted in numerically higher 4% FCM and ECM efficiencies for cows received AA supplementation. There was a statistical interaction of treatment and day observed on DMI, OMI, and 4% FCM and ECM production efficiencies indicating that the treatment group maintained a better production efficiency than the control group throughout the experiment. The concentration of ruminal NH4+ and the proportion of butyrate were increased, but proportions of acetate and valerate were decreased while the pH, total VFA, proportions of propionate, isobutyrate and isovalerate, and the ratio of acetate to propionate were not affected by supplementing AA.