In six groups of nine young Sprague-Dawley rats each, the coconut fat used in the basal diet (group I) was replaced to 60% by pure oleic acid (group II) or linoleic acid (group III), respectively, by 0.6% of alpha-linolenic acid (IV), eicosatrienoic acid (V) and eicosapentaenoic acid (VI). A 60% replacement of the coconut fat in the basal diet by pure oleic acid, respectively, by 0.6% of eicosapentaenoic acid or eicosatrienoic acid significantly decreased the total cholesterol concentration in serum by 21% (HDL -28%; LDL -15%; VLDL -48%), respectively, 19% (HDL -12%; LDL -42%; VLDL +69%) or 15% (HDL -11%; LDL -21%) and the total free cholesterol concentration by 22% (HDL -27%), respectively, 41% (HDL -31%; LDL -66%) or 23% (HDL -21%; LDL -31%). The substitution of the coconut fat by 60% oleic acid or 0.6% alpha-linoleic acid had no influence on the total and total free cholesterol concentration in serum, though in the LDL-fraction a significant decrease of cholesterol could always be found. The concentrations of free fatty acids in serum were not influenced by the unsaturated dietary fatty acids while the phospholipid concentration was reduced by linoleic acid (-15%) and by eicosapentaenoic acid (-18%). The triglyceride content in serum was decreased by linoleic acid (-29%) and by eicosatrienoic acid (-25%).