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Little quantitative information is available on how dietary lipids concurrently alter the main rumen microbial functions in relation with their incorporation level. In a three-period experiment, linseed (Linum usitatissimum, LO) and rapeseed (Brassica napus L., RO) oils were added at 0, 40 or 80 g/kg dry matter input (DMI) to five 1-L dual outflow fermenters that were on a maize silage-based diet for nine days per period. RO supply decreased butyrate specific production. The amount of hexoses fermented (HF) increased by 9% at 40 g/kg LO. The production of CH4 was lower at 80 g/kg LO by 46% compared with controls. Conversely, the supply of LO significantly increased H2 and H2S productions in an antagonistic mode. The specific productions of propionate, butyrate, CH4 and H2 were altered by LO. The supply of RO increased the ammonia daily outflow (by 23% at 40 g/kg) and decreased the organic N outflow (by 13% at 80 g/kg). The degradabilities of dietary fractions were not affected by RO, neither the OM partitioning between dietary, fermented and microbial outflows. The OM true degradability decreased at 80 g/kg LO compared with controls and 40 g/kg LO. When LO was supplied, isovalerate and ammonia-N outflows were higher, organic and microbial N outflows and EMPS were lower along with changes in the OM outflow partitioning. Overall microbial processes appeared to differ in their responses to fatty acids saturation. Moreover, most effects were present at 40 g/kg DMI and diminished or even plateaued at 80 g/kg DMI.Highlights Four per cent oil lowered butyrate specific production Four per cent Linseed oil lowered microbial protein synthesis efficiency Linseed oil lowered methanogenesis in proportion to input level