The experiment was carried out in a Rusitec system (rumen simulation technique). The effect of a high forage diet (60% fresh lucerne + 40% maize) and different fat sources (linseed LO, rapeseed RO, fish FO oils, 5% wt.wt^-1) on rumen fermentation was studied. The experiment lasted 13 days. To ensure a steady-state within the fermentation Rusitec vessels a 7-day adaptation period preceded the 6-day collection period. Fermentation vessels were supplied daily with the high forage diet (control) and a 5% addition of LO, RO or FO. Supplementing oil (LO, RO, FO) did not affect some of the basal parameters of rumen fermentation (pH, total VFA production, dry matter and detergent fibre digestibility) in comparison with the control. The oils significantly reduced the mol% of acetate (LO, FO; P < 0.001; RO; P < 0.05), n-butyrate (FO; P < 0.01), and A/P ratio (LO, FO; P < 0.001) and increased the mol% of propionate (LO, FO; P < 0.001). Rapeseed oil supplementation slightly affected these parameters of rumen fermentation. The oils (LO, FO, RO) supplemented to the high forage diet had no effect on the efficiency of microbial protein synthesis.