The effect of rape seed oil on net microbial protein synthesis and other metabolic parameters was estimated on four rams fitted with rumen cannulas in a 4 x 4 Latin square design consisting of four diets differing in the percentage of rape seed oil supplement. The basal diet was composed of 50% concentrate and 50% meadow hay and was supplemented with 4, 8 or 10% of rape seed oil. Supplementation of the diet with 4 and 8% of rape seed oil caused a drop in net microbial protein production, determined by allantoin excretion in urine, but the differences were not significant. The highest dose of rape seed oil had no effect on the intestinal flow of microbial protein. Fat supplement had no influence on pH and ammonia-N or on total VFA, acetate and propionate concentrations. The butyrate concentration was, however, lower (P<0.05) when rape seed oil was added to the basal diet.