Experiments with red deer and sheep compared sunflower seed supplementation and species effects on tissue fatty acid composition and ruminal fatty acid biohydrogenation. An efficient lipid extraction method was developed allowing a large number of small samples to be analysed using small volumes of hexane:isopropanol. In Experiment 1, groups of 12 female red deer and sheep grazing ryegrass either received sunflower oil or no oil. Oil supplementation increased trans-vaccenic acid (TVA) concentrations in adipose tissues. Muscle had higher TVA and conjugated linoleic acid (CLA) ratios [relative to stearic acid (CI8:0)] in sheep, and higher total polyunsaturated fatty acid (PUFA) ratios, including linoleic (CI8:2) and linolenic (CI8:3) acids, in deer. Different supplementation effects of species, and tissues suggested metabolic differences. In Experiment 2, groups of eight red deer stags and castrated sheep received grass hay and concentrate, supplemented with sunflower meal, sunflower oil or no supplement. The highest C18:0 proportions (relative to total fatty acid levels) were found in tissues from sunflower meal-supplemented animals, possibly due to more biohydrogenation of lipid· in sunflower meal than of oil. Tissue proportions of TVA and CLA were higher for oil-supplemented than meal-supplemented animals. In both experiments gut passage rates were higher in red deer than in sheep. In Experiment 3, incubation of buffered rumen liquor from red deer and sheep, with substrates of dried grass and grass' + sunflower meal, showed rapid biohydrogenation of fatty acids. Compared with sheep rumen liquor, deer liquor had higher contents of CI8:0, CI8:2, C18:3 and CLA, and appeared to have a higher biohydrogenation capacity. ,. The higher PUFA ratio in deer tissues compared with sheep may result from faster gut passage rate and other physiological differences. Sunflower seed supplementation can alter PUPA proportions in ruminant tissues and thus meat quality. Relatively high PUPA and low saturated fatty acid contents suggest venison as a healthy meat for human diets. Keywords: Red deer, sheep, sunflower supplementation, PUFA, TVA, CLA, biohydrogenation