Forty-eight (Wistar) rats were divided into 3 groups and fed semi-synthetic diets containing various types of fat (6%): rape seed, linseed, or fish oil. Half of the animals in each group received lecithin (5% of the mixture). The content of fatty acids in the serum and m. longissimus dorsi was determined, as well as serum triacylglyccrides (TAG) and total cholesterol (TCh). It was found that feeding fish oil increased the percentage of eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids in the serum and muscle tissue, while the addition of linseed oil increased the content of α-linolenic acid and EPA in these tissues. The addition of lecithin raised the percentage of serum linoleic acid and decreased, EPA and DHA. TAG and TCh concentrations were much lower in the animals receiving lecithin and fish oil in their diets and, to a lesser degree, also in those receiving linseed oil, in comparison with rats fed rape seed oil.