Summary. Influence of lnoleic acid and a-linolenic acid on the composition, permeabi7ity and fluidity of cardiac phospholipids in rats : a study using membrane models (liposomes).For 9 weeks, 80 male SPF Wistar rats were fed purified diets containing mixtures of vegetable oils (15 % by weight) with different linoleic and linolenic contents. Diet (L·) contained large amounts of linoleic acid (53 % of the total fatty acids), diet (L + Ln) contained the same amount of linoleic acid but also 10 % of a-linolenic acid, diet (L-) supplied a low level of linoleic acid (12 % of the total fatty acids) and so did diet (L-Ln) which also contained 10 % of a-linolenic acid. The levels and fatty acid composition of heart phospholipids were determined. Liposomes, prepared from the total phospholipids extracted from rat hearts, were tested at different temperatures (15 to 50 °C) for their permeability to urea and fluidity ; fluidity was monitored by fluorescence depolarization of 1,6-diphenyl-1,3,5-hexatriene (DPH). ).As already demonstrated, dietary a-linolenic acid substituted C22 polyunsaturated fatty acids of the linoleic family (n-6) for those of the linolenic acid family (n-3). This substitution remained high, even though linoleic acid represented more than 50 % of the total dietary fatty acids. Diphosphatidylglycerol markedly increased in heart phospholipids of rats fed diets (L + ) and (L + Lnl. The changes in liposome permeability observed were rather well correlated with the unsaturation index of the phospholipid fatty acids, whereas fluidity changes were not. Fluidity decreased in liposomes of rats fed high levels of linoleic acid. Factors such as diphosphatidylglycerol or (n-3) fatty acid concentration in heart phospholipids could explain these results.Introduction.