Ruminants have a low efficiency of nitrogen (N) utilization that has negative implications for animal production and the environment, but reducing the ruminal degradation of protein can help to reduce N losses. The objective of this study was to evaluate the inclusion of sunflower meal (SM) and sunflower seed (SS) protected against ruminal degradation in high-cereal diets on in vitro ruminal fermentation and CH4 production. Samples of SS and SM were sprayed with a solution of malic acid 1 M (400 ml/kg sample) and dried at 150°C for 1 hr as a protective treatment. Four diets were formulated to contain either 13 (low) or 17 (high) g of crude protein (CP)/100 g dry matter (DM), and included SM and SS either untreated (13CON and 17CON diets) or treated as before described (13TR and 17TR diets). Diets were incubated in vitro with rumen fluid from sheep for 8 and 24 hr. The treatment did not affect (p ≥ 0.57) total volatile fatty acid (VFA) production at any incubation time, but it reduced (p < 0.05) NH3-N concentrations by 19.2 and 12.5% at 8 and 24 hr respectively. Both CH4 production and CH4/VFA ratio were lower (p < 0.02) in TR than in CON diets at 8 hr, but differences disappeared (p > 0.05) at 24 hr. The treatment increased the molar proportion of propionate (p = 0.001) and reduced that of isovalerate (p = 0.03) at 8 hr compared with CON diets, but only a reduction of isovalerate proportion (p = 0.03) was detected at 24 hr. There were no treatment x crude protein level interactions (p > 0.05) in any parameter, but high-protein diets had greater NH3-N concentrations (p < 0.001) and lower VFA production (p < 0.001) than low-protein diets at 24 hr. The treatment reduced protein degradation, and CH4 production was decreased by 4.6 and 10.8% for low- and high-protein diets, respectively, at short incubation times without affecting VFA production, thus improving fermentation efficiency and decreasing polluting emissions.