The experiments were carried out in a 4 x 4 Latin square design on 4 rams (40 ± 5 kg) fitted with permanent rumen cannulas to determine the effects of fat source on basic rumen parameters. The basic ration for the control group consisted of meadow hay and concentrate (60:40) and was supplemented for experimental groups with rape seed oil, hydrogenated rape seed oil, or linseed oil at a level of 0 (control), 4, 8 and 10% in dry matter of concentrate. Samples of rumen fluid were analyzed for ammonia, volatile fatty acid (VFA) concentrations, and pH. VFA profiles, except of the level of isovaleric acid, were not influenced by the type of fat when rape seed oil or hydrogenated rape seed oil was added to sheep rations (P < 0.05; P < 0.01). A significant (P < 0.05; P < 0.01) decrease in the ammonia level was found when 4, 8 and 10% of rape seed oil was supplied to the ration. The addition of rape seed oil caused an increase (P < 0.01) in pH in the group with 10% fat supplementation. Hydrogenated rape seed oil and linseed oil have no influence on the rumen pH level. Diets for ruminants can be supplemented with up to 10% of vegetable origin fat as an additional energy source without altering ruminal fermentation when the proper forage: concentrate ratio is maintained.