The effect of different types of fat supplementation on basic rumen parameters was estimated on three milking sheep fitted with rumen cannulae in a 3x3 Latin square design consisting of three experimental diets differing in the percentage of fat supplement. 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, soyabean oil, linseed oil, tallow, Bergafat^® or fish oil at a level of 0 (control), 4, and 6% in dry matter. Samples of rumen fluid were analyzed for volatile fatty acids (VFAs), ammonia (N-NH₃) and pH. The acetic acid level increased significantly (P<0.05) when 4 and 6% of soyabean oil was added to the ration. Similarly, addition of linseed oil increased (P<0.05) the level of acetic acid in experimental groups. A significant (P<0.05) increase in the acetic acid level was observed when 6% Bergafat^® was added, whereas the level of this acid was reduced when 6% fish oil was added to the diet. The butyric acid level decreased significantly (P<0.05) when the diet contained 6% linseed oil, and there was a decrease (P<0.05) in isobutyric acid in both groups receiving fish oil. A slight but statistically significant (P<0.05) increase in the isobutyric acid level when the diet was supplemented with 4% soyabean oil was also observed. The addition of rape seed oil and tallow had no influence on fatty acid levels in the experimental groups. There were no main treatment effects on daily mean pH and ammonia concentration in the rumen. With all fat additives, except for fish oil, the ruminal N-NH₃ concentration was lower in both experimental groups, but the differences were not significant. Only the addition of 4% fish oil to the ration was accompanied by a significant (P<0.05) increase in the ammonia level.