An experiment was conducted to study the effects of supplementary extruded cotton seed (ECOS) or canola seed (ECAS) on the composition of unsaturated fatty acids in the plasma, erythrocytes and livers of Mehraban male lambs. The treatments included: (1) control (C); (2) diet C+6% ECAS, (3) diet C+6% ECOS, (4) diet C+12% ECAS, (5) diet C+12% ECOS, (6) diet C+6% ECAS+6% ECOS, (7) diet C+12% ECAS +6% ECOS, (8) diet C+6% ECAS +12% ECOS, (9) diet C+12% ECAS+12% ECOS, (10) diet C+18% ECAS+18% ECOS. A completely randomized design experiment was applied to feed the 10 dietary treatments to 60 individually pen housed lambs, in order to have six lambs (replicates) per diet. The average weight of 5 - 6 months lambs at the onset of the study was 34.3 ± 2.12 kg. The experiment lasted for 90 days. Results indicated that the amount of oleic acid (C18:1), linoleic acid (C18:2 n-6) and linolenic acid (C18:3 n-3) in plasma and oleic acid, and linolenic acid in erythrocyte lipids were higher in lambs fed diets containing either or a combination of supplemental extruded oil seeds compared to those fed the control diet. The supplementation of 6% or 12% ECAS significantly increased linoleic acid in liver lipids compared to those fed the diet containing ECOS. There was a linear correlation between the levels of dietary ECAS or ECOS with linoleic acids content of the liver. This study provides evidence that dietary oleic acid, linoleic acid and linolenic acid might escape biohydrogenation in the rumen and showed that the type of dietary fat has a marked impact on lipid metabolism in the liver
