The influence of linseed oil (LO) and selenium (as Na₂SeO₄) on organs, muscles and body weight gain of sheep, as well as the fatty acid composition of M. longissimus dorsi was investigated. Twenty male lambs of body weight of 25±2 kg were divided into four treatment groups and fed a control diet or experimental diets enriched in 5% linseed oil (LO), 2 ppm selenium (SE) or combined addition of 5% LO and 2 ppm selenium (LOSE). The effects of the experimental diets on organs and body weight gain of the sheep were small, but the LOSE treatment increased M. biceps femoris mass and body weight gain. All experimental diets numerically decreased the level of saturated fatty acids as well as the content of cis-9C18:1 in M. longissimus dorsi. LO supplied to the diet numerically increased the content of conjugated linoleic acid isomers and other unsaturated fatty acids in M. longissimus dorsi (with the exception of cis-9C18:1). The SE and LOSE treatments numerically decreased the content of unsaturated fatty acids.