In the rat, the effect of the bile duct ligation on liver microsomal phospholipid fatty acid composition and on phosphatidylcholine, phosphatidylserine and phosphatidylinositol pattern were studied. After two days of cholestasis, microsomal phospholipid fatty acids showed a decrease in linoleic, stearic and arachidonic acids and an increase in oleic and docosahexaenoic ones, as compared to controls. Phosphatidylcholine showed an increment in oleic and palmitic acid content and a concomitant decrease in arachidonic acid. Phosphatidylserine showed a progressive increase while phosphatidylinositol showed a progressive decrease in all fatty acids. Eight-days post-cholestatic rats showed a marked increase in oleic acid, whereas linoleic, arachidonic, stearic and palmitic acids concentration decreased. Phosphatidylcholine showed a global decrease in its fatty acid content, except for oleic which is increased. Phosphatidylserine showed an increase over the two-days cholestasis fatty acids values. Phosphatidylinositol decreased in most fatty acids except in docosahexaenoic acid that recovered normal values. It was concluded that cholestasis produced significative changes in the fatty acid composition of the major phospholipids constituents of the microsomal membranes.