The objectives of this trial were to determine the effects of increasing doses of sodium monensin on rumen metabolism of lambs fed high-forage diets. Five wethers (BW 70.5 ± 2.8 kg), cannulated in the rumen, were used in 5 × 5 Latin Square design. Animals were fed daily and diet was composed of coastcross hay. Sodium monensin were offered twice a day and doses were 0 (control), 8, 16, 24 or 32 mg/kg DM, corresponding to 0, 40, 80, 120 and 160 mg of Rumensin 200. The delivery vehicle of the set dosage of monensin was 20 g of ground corn per 1 kg DM intake. Every period of experiment lasted 20 d and rumen fluid was collected in the last day, every 3 h, starting prior feeding, 3, 6, 9 and 12 h after feeding. Short-chain fatty acids (SCFA) and pH were analyzed as repeated measures over time. Protozoa concentration was determined only 3 h after feeding. Data were analyzed using MIXED procedure (SAS Inst. Inc.) and LSMEANS option was used to generate individual means. Orthogonal polynomials for diets responses were determined by linear and quadratic effect. There was a quadratic response for acetate (78.22, 78.25, 78.65, 77.62, 76.27 mM/100 mM, P = 0.03), propionate (14.81, 14.59, 15.07, 15.60, 16.90 mM/100 mM, P = 0.02), isobutyrate (0.66, 0.59, 0.53, 0.56, 0.64 mM/100 mM, P = 0.02) and acetate:propionate ratio (5.30, 5.35, 5.24, 5.02, 4.54, P = 0.02). However, there was an interaction (P < 0.05) between diet × hour for acetate:propionate ratio. The inclusion of sodium monensin decreased acetate:propionate ratio during 9 and 12 h. There was a linear response for valerate (0.76, 0.66, 0.59, 0.60, 0.59 mM/100 mM, P = 0.03) and pH (6.57, 6.61, 6.46, 6.43, 6.49, P = 0.05). Butyrate (4.42 ± 0.06 mM/100 mM), isovalerate (1.01 ± 0.04 mM/100 mM), total SCFA (85.58 ± 1.14 mM/L) and protozoa concentration (1.88 ± 0.22 × 10⁵/mL) were unaffected (P > 0.05) by the experimental diets. Sodium monensin doses above 16 mg/kg DM provided an increase in the molar proportion of propionate over acetate, consequently reducing the acetate-to-propionate ratio and pH.