An experiment lasting 13 days was carried out using a Rusitec system (rumen simulation technique). The effect of a forage diet (100% fresh lucerne – control) and sources of fat supplements (linseed LO, rapeseed RO, fish FO oils, 5% wt.wt^-1) on rumen fermentation was studied. To ensure a steady-state within the fermentation control vessel, a 7-day adaptation period preceded the 6-day collection period. Every day portions of a fresh lucerne diet with a 5% addition of LO, RO or FO were supplied to the fermentation vessels. Oil supplementation (LO, RO, FO) did not affect the basal parameters of rumen fermentation (pH, total VFA production, dry matter and detergent fibre digestibility) in comparison with the control. The oils, mainly LO and FO, significantly reduced the mol% of acetate and n-butyrate, and increased the mol% of propionate, while rapeseed oil only slightly (NS) affected these parameters of rumen fermentation. The oils (LO, FO, RO) supplemented to the forage diet did not affect the efficiency of microbial protein synthesis.