A microanalytic method for determining fatty acid composition in rat plasma was adapted for gas-liquid chromatography (GLC) equipped with capillary column. Nonàdecanoic acid (C19:0) was used as the internal standard for calculation. The coefficients of variation, as an evaluation of reproducibility, ranged under 4.7% for myristic (C14:0), palmitic (C16:0), palmitoleic (C16:1), stearic (C18:0), oleic (C18:1, n = 9), vaccenic (C18:1, n = 7), linoleic (C18:2), linolenic (C18:3), arachidonic (C20:4) and docosahexaenoic (C22:6) acids. The recovery of the above mentioned fatty acids was at least 91%. The present analysis method made it possible to discriminate the C18:1, isomers into oleic and vaccenic acids among the above higher fatty acids, so the method will be very useful for determination of fatty acid compositions of dietary foods and biological samples. The changes in fatty acid composition with age were investigated by the analysis of plasma lipids of young, adult, and old rats 7, 40, and 80 weeks of age, respectively. The proportions of stearic, docosahexaenoic and vaccenic acids increased significantly with age, and myristic, linoleic and linolenic acids decreased. The ratio of unsaturated fatty acid to saturated fatty acid decreased significantly with age. The authors concluded that this microanalytic method was useful for separating C18:1 isomers and it was found that vaccenic acid in rat plasma increased with age.