The experiment was carried out in a Rusitec system. The effect of diet (control-fresh lucerne plus maize, 60:40%) and fat sources (linseed LO, rapeseed RO, fish FO oils) as oil blends (LO+RO,FO+LO,FO+LO+RO, 5%wt.wt^-1) on rumen fermentation was studied. The experiment lasted 13 days. To ensure a steady state within the Rusitec vessels a 7-day adaptation period preceded a 6-day collection period. The diet with a 5% addition of oil blends, LO+RO, LO+FO, LO+FO+RO, was added to the fermentation vessels daily. Supplementation with blends (LO+RO, LO+FO, LO+FO+RO) did not affect some basal parameters of rumen fermentation (pH, total VFA production, dry matter digestibility) in comparison with the control. Detergent fibre digestibility was significantly reduced, mainly with LO+RO+FO (NDF, ADF, cellulose) and LO+RO (ADF). The oil blends significantly reduced the mol% of acetate, n-butyrate, A/P ratio, and increased the mol% of propionate. The oil blends supplemented to the high forage diet significantly (P < 0.005)decreased the efficiency of microbial protein synthesis.