SummaryThe effect of including full-fat rapeseed (Brassica napus, cv. Jet Neuf) into diets for mature wether sheep was studied. A 4 × 4 latin square was carried out with diets containing 4% rapeseed oil in different physical forms: as intact seeds; as a mixture of rapeseed oil and rapeseed meal; or as an extruded rapeseed product. A control diet, containing rapeseed meal but no added oil, was also fed.There was no effect of added oil on rumen fermentation or dietary fibre digestibility. Extrusion had the effect of lowering nutrient availability, both in the rumen and in the small intestine. Total nitrogen digestibility for the diet containing the extruded product was 5% lower than any other diet.All diets produced apparent net gains between mouth and duodenum of ether extract; however, the diet containing dispersed oil and meal was the only one that did not show apparent synthesis of long-chain fatty acids in the forestomach. Apparent disappearances of fatty acids in the small intestine were highest, at 56 and 58 g/day respectively, for the diets containing the extruded product and the intact whole seeds. Extrusion had the effect of reducing apparent biohydrogenation of 18:1 in the forestomach.The use of extruded rapeseed in this experiment showed little benefit over that of intact rapeseed, for sheep, owing to the lower digestion of nutrients for the heated supplement. The free-dispersal of rapeseed oil in the diet, however, may inhibit microbial synthesis in the rumen.