Some studies have shown a synergistic effect of supplementing dairy cow diets with a blend of fish and soya oil on the concentration of conjugated linoleic acid (CLA) in milk. This is apparently due to increased accumulation of ruminal and tissue concentrations of vaccenic acid (VA), the main substrate for ∆-9 desaturase catalysed, de novo tissue synthesis of the cis 9, trans 11 isomer of CLA. Furthermore, recent in vitro studies suggest that the dietary ratio of linoleic acid to the omega-3 polyunsaturated fatty acids (ω-3 PUFA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) may be an important mediator of this synergism (AbuGhazaleh and Jenkins, 2004). Additionally, dietary supplementation with ω-3 PUFA can shorten the shelf life of meat. The objectives of the current study were to examine level and duration of dietary PUFA supplementation on tissue fatty acid concentrations and on lipid oxidative stability of beef cattle.