The effect of different types of fat supplementation on the level of purine derivatives in urine and bacterial protein synthesis in the rumen was estimated on four rams in a 4 x 4 Latin square design with four experimental diets differing in the percentage of supplemented fat. 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. Allantoin excretion in urine was used as a marker for estimation of microbial protein synthesis. Bacterial protein synthesis declined (P<0.05) in response to an increased concentration of rape seed or linseed oil in the diet. Hydrogenated rape seed oil had no influence on either microbial protein synthesis in the rumen or on the level of purine derivatives in urine. The allantoin level in urine and hence bacterial synthesis in the rumen decreased when 8 and 10% of rape seed oil was added to the ration. The level of uric acid, hypoxanthine and xanthine in urine was unaffected by the type of added fat.