Greenland Eskimos who live on a traditional marine diet rich in long chain omega 3-polyunsaturated fatty acids have a low incidence of cardiovascular disease and myocardial infarction. In their plasma and platelet lipids, arachidonic acid, the precursor of dienoic prostanoids, is partly replaced by eicosapentaenoic acid (C20:5, omega 3; EPA), the precursor of trienoic prostanoids. Studies with an Eskimo diet or a Western diet supplemented with sea fish or fish oil rich in EPA resulted in an 'Eskimo-like' pattern of plasma and platelet lipids. Moreover, less reactive platelets, a reduced ex vivo formation of proaggregatory thromboxane A2 and a blunted circulatory response to pressor hormones were reported. These favourable functional effects may be induced by a shift of prostanoid formation from the dienoic to the trienoic series. We show here that the major urinary metabolite of endogenous prostaglandin I3 is present in subjects that have ingested either cod liver oil (approximately 4 g EPA per day) or mackerel (approximately 10-15 g EPA per day). Our studies provide the first direct evidence for in vivo formation of prostaglandin I3 in man.