The experiment was carried out on Polish Longwool Sheep wethers with rumen cannulas in a 4×4 Latin square design to determine the influence of fat source and amount in the ration on nutrient digestibility and rumen fermentation. The basic ration for the control group (C) consisted of meadow hay and concentrate. For the experimental groups, this ration was supplemented with linseed (L), linseed oil (OL) and Dunafett (D). Rumen fluid was taken from each animal 0; 1.5; 3.0 and 4.5 h after feeding. There was no influence of the type and amount of vegetable fat on rumen fermentation and digestibility of nutrients. There were, however, significant differences in the urine allantoin level among the groups.