With respect to human health, nutritional guidelines emphasise on increasing the ratio poly-unsaturated/saturated fatty acids (P/S) (> 0.7) and reducing the n-6/n-3 ratio (< 5). Beef is generally characterised by a low P/S ratio (0.1), while the n-6/n-3 ratio may vary between 2 and 10 depending on the feeding strategy. Hence, the provision of n-3 fatty acids by the diet is crucial for optimising the fatty acid composition of meat. However, PUFA are hydrolysed and subsequently hydrogenated in the rumen to more saturated fatty acids and intermediates that are absorbed from the intestinal tract and transported to the tissues. Lipolysis and hydrogenation may thus affect the meat fatty acid composition. For ruminants, important sources of linolenic acid (C18:3n-3) are linseed and fresh grass. Prior to use in animal nutrition, linseed needs physical treatment like extrusion or crushing. The aim of the present in vitro experiment was to study lipolysis and biohydrogenation of linoleic acid (C18:2n-6) and C18:3n-3 in fresh grass and extruded or crushed linseed products.