-The isomerization of linolenic acid into mono-and di-trans isomers takes place during heat treatment. One of these compounds, l8:3ð.9c, l2c, 15t can be desaturated and elongated to form, in particular, the trans isomers of eicosapentaenoic and docosahexaenoic acids. This study was undertaken to observe the incorporation of these trans long-chain n-3 polyunsaturated fatty acids into the lipid classes of several tissues. Rats were fed for 8 weeks with heated linseed oil containing high levels of trans linolenic acid isomers. The lipid classes of the liver, kidney, heart and adrenals of these rats contained various levels of these compounds. The 20:5A5c,8c,1 c,14c,17t represented between 80 and 90 % of the total 20:5 in the phospholipids, but 22:5A7c, I Oc.l3c.l6c, 19t and mainly 22:6ð.4c,7c,lOc,13c,16c,19t