The effect of rape seed oil supplementation on efficiency of microbial protein synthesis was estimated on three rams fitted with rumen cannulae in 3x3 Latin square design consisting of three experimental diets differing in the percentage of rape seed oil supplement. The basic diet for all animals was composed of 60% concentrate and 40% meadow hay which was supplied with 0. 3 or 6% of rape seed oil. Supplementation of the diet with 3 or 6% of rape seed oil did not have a significant effect on rumen pH and total volatile fatty acid concentrations (P<0.05). There was a statistically significant difference in the level of valeric acid among groups. Also the level of ammonia-N was altered significantly. Daily output of microbial protein-N, determined from allantoin content in the urine, decreased with the amount of fat added to the ration from 15 to 13 and 12 g, respectively, but these differences were not statistically significant.