Linoleic and linolenic acids are precursors for the biosynthesis of prostaglandins (PG) and related compounds. In man augmented linoleic acid intake results in stimulation of PG-biosynthesis, for PG originating from the kidney a stimulation only is found for PGE2. Dietary linolenic acid inhibits PG biosynthesis to a tenfold lesser degree than eicosapentaenoic acid. Renal PGE2 is depressed by linolenic acid intake, while no effect is found for PGF2 alpha up to 8 energy% of linolenic acid supply. Linoleic and linolenic acids additionally display effects on fatty acid metabolism. In contrast to the results of in vitro studies the supply with the precursor linoleic acid results in a decrease of arachidonic acid in cholesterolesters of plasma and in HDL-lecithin, while the intake of linolenic acid is without effect. From these data it is concluded that in vivo the conversion of linoleic to arachidonic acid occurs preferentially to the analogous conversion of linolenic to eicosapentaenoic acid.