The experiments were carried out in a 4 x 4 Latin square design on 4 rams of 40±5 kg body weight, fitted with permanent rumen and duodenal cannulas, to determine the influence of fat source and amount in the ration on the extent of C18 unsaturated fatty acid hydrogenation in the rumen. The basic ration for the control group consisted of meadow hay and concentrate (60:40). For the experimental groups this ration was supplemented with vegetable fat in the form of rape seed oil, hydrogenated rape seed oil, or linseed oil at a level of 0 (control), 4, 8 or 10% in dry matter of the diet. There was no influence of the type and amount of vegetable fat on the level of biohydrogenation except in the diet containing 10% hydrogenated rape seed oil, which resulted in increased (P < 0.05) C18 unsaturated fatty acid hydrogenation in the rumen of sheep.