Effects of extracts derived from the husk of Lafoensia pacari (27.5% tannins, mainly hydrolysable) and Stryphnodendron adstringens (52.2% tannins, mainly condensed) on rumen fermentation were evaluated in 2 independent in vitro experiments. The doses were CTL, LOW, MEDIUM and HIGH (0, 30, 300, and 3000 mg/L of buffered rumen fluid, respectively). The diet (50:50 concentrate:roughage, 91% DM, 20% CP, 1.6% EE, 39% NDF) was ground (1 mm) and incubated using filter bags for 3, 6, 12, 24, 48, 72 and 96 h. The rumen fluid donor was fed with a 50:50 diet. The incubation was repeated 4 times in each experiment. The effective degradability (ED, passage rates of 5 and 8%/h) and potential degradability (PD) of DM were calculated based on DM disappearance. Culture fluid was sampled after 96 h of incubation. The responses of CTL, LOW, MEDIUM and HIGH doses of L. pacari were, respectively: ED5%/h = 51, 50, 50 and 28% (SEM = 0.75, P < 0.01); ED8%/h = 44, 43, 42 and 26% (SEM = 0.69, P < 0.01); DP = 81, 82, 81 and 48% (SEM = 2.57, P < 0.01); N-NH₃ = 12.3, 11.7, 9.8 and 6.5 mg/L (SEM = 0.88, P < 0.01); Total VFA = 62.9, 57.6, 50.9 and 42.3 mM (SEM = 4.28, P = 0.03); Acetate = 40.6, 37.1, 33.5 and 27.3 mM (SEM = 2.02, P = 0.01); Propionate = 11.9, 11.2, 9.7 and 8.4 mM (SEM = 0.81, P = 0.05). The HIGH dose reduced ED5%/h (46%), ED8%/h (41%) and PD (40%), compared with CTL. The N-NH₃, total VFA, acetate and propionate decreased with HIGH and also with MEDIUM and LOW doses, which were similar. In the S. adstringens study, responses of CTL, LOW, MEDIUM and HIGH doses were, respectively: ED5%/h = 51, 52, 52 and 25% (SEM = 1.59, P < 0.01); ED8%/h = 44, 45, 44 and 23% (SEM = 1.58, P < 0.01); DP = 81, 82, 83 and 37% (SEM = 1.74, P < 0.01); N-NH₃ = 10.2, 10.6, 9.8 and 6.4 mg/L (SEM = 0.83, P = 0.01); Total VFA = 55.7, 58.6, 52.8 and 32.7 mM (SEM = 4.40, P < 0.01); Acetate = 36.8, 37.1, 33.5 and 20.9 mM (SEM = 2.63, P < 0.01). The HIGH dose reduced the ED5%/h (52%), ED8%/h (48%), PD (54%), total VFA (41%) and acetate (43%), compared with CTL. CTL and LOW doses showed similar concentration of N-NH₃, which decreased with MEDIUM and HIGH doses. There was no effect on propionate (P = 0.13). In conclusion, these plant extracts can be potentially useful to modify rumen fermentation patterns.