The objective was to study the effect of the typical plant extracts of the Brazilian Cerrado on in vitro rumen microbial fermentation under a high concentrate diet. Two separate trials were conducted to evaluate 4 doses (0, 30, 300 and 3,000 mg/L) of stem bark extract from Stryphnodendron adstringens (Trial 1; 48.1% tannins) and Lafoensia pacari (Trial 2; 21.1% tannins). The trials were analyzed as a randomized block design with 5 repetitions, treating incubation as the blocking factor. Treatments were evaluated using a TE-150 incubator ((Tecnal) in a batch culture of rumen fluid with a 10:90 forage:concentrate diet (15% crude protein, 20% neutral detergent fiber) incubated using filter bags of non-woven textile for 2, 4, 8, 12, 24, and 48 h (triplicates each time) to fit the disappearance of dry matter (DM). Volatile fatty acids (VFA), pH, and ammonia nitrogen (NH₃-N) were evaluated after 48 h of incubation. S. adstringens extract at 3,000 mg/L reduced (P < 0.05) in 34.5% the potential degradability (a + b) of DM. There was significant reduction in total VFA concentration from 66.3 mM (0 mg/L) to 40.2 and 24.3 mM by adding 300 and 3,000 mg/L of S. adstringens extract, respectively. This was also observed for NH₃-N concentration, representing a decrease of 36.9% (300 mg/L) and 57.8% (3,000 mg/L).The pH significantly increased (5.9 vs 6.1) with the addition of 3,000 mg/L. Regarding L. pacari extract, the addition of 3,000 mg/L promoted (P < 0.05) reduction of 20.3% in the potential degradability of DM but detrimental effects on the fermentation profile were not observed. The 30 mg/L dose increased (P < 0.05) the total VFA concentration (63.4 vs. 85.2 mM), decreased the acetate:propionate ratio (4.3 vs 3.6) and the proportion of isovalerate (1.9 vs 1.5 mM). Changes in the concentration of NH₃-N were only observed at a dose 3,000 mg/L, with a reduction of 43.0%. The results indicated that Cerrado plant extracts are potential modifiers of rumen fermentation under a high concentrate diet.