Nine rumen-fistulated cows were used in a completely randomized design replicated three times to evaluate three monensin (0, 150 and 300 mg/animal/day) and three concentrate levels (25, 50 and 75%) in a 3x3 factorial arrangement. Twenty-one day subperiods were used, the first sixteen for diet (Tifton hay+concentrate) adaptation. Monensin decreased DMI only in 50%-concentrate diet (P<0.05) and did not influence total VFAs concentrations. For ruminal pH, molar% of VFAs, and NH₃-N concentration collected 7 times/day, the responses to high level of monensin were higher in low-forage diets (P<0.05). Responses to low level of monensin were higher in high-forage diets (P<0.05).