The effects of sodium monensin and protein supplements on volatile fatty acid production were studied in a 4 x 4 Latin Square design, using four female bovines equipped with rumen fistulae and fed Brachiaria brizantha Marandu cultivar hay. The treatments were organized in a 2 X 2 factorial array (with or without 160 mg of sodium monensin/animal/day and with or without protein supplements). Protein supplementation increased the concentration of acetic acid and sodium monensin reduced the concentrations of acetic and butyric acids and the acetic : proprionic ratio, both with and without protein supplements, except for the material collected at 8 h. Sodium monensin reduced the total concentration of fatty acids only in the material collected at 0 h, while protein supplementation increased the concentration of fatty acids at 4, 6 and 8 h. Used together, they increased the molar concentration of propionate in the rumen fluid, without decreasing the total concentration of fatty acids.