Nine healthy male volunteers were given 15 Max EPA fish oil capsules providing 2.67 g of eicosapentaenoic acid (EPA, 20:5 omega 3) and 1.72 g of docosahexaenoic acid (DHA, 22:6 omega 3) daily for 3 wk. Measurements were taken at baseline, at the end of the fish-oil period, and at 2 and 6 wk postsupplementation. The effect of fish oil on plasma lipids and the fatty acid composition of individual platelet phospholipids was studied. In general, the proportions of 20:5 omega 3 and 22:6 omega 3 in platelet phosphoglycerides were substantially increased mainly at the expense of arachidonic acid (AA, 20:4 omega 6). A large and significant increase in the relative EPA content of phosphatidylcholine (PC) (P less than 0.001) and phosphatidylethanolamine (PE) (P less than 0.001) was noted at the end of the 3 wk supplementation. We have also shown for the first time a small but significant (P less than 0.001) incorporation of EPA in phosphatidylserine (PS). Incorporation of DHA was also detected in PC, PE and PS, whereas the relative AA content of these phospholipids was significantly reduced. Fish oil supplementation led to a significant increase of 22:5 omega 3 in PS and decreases of 20:3 omega 6 in PC and 22:4 omega 6 in PE. Postsupplementation measurements showed a gradual return of all fatty acids to baseline levels. The fatty acid composition of the phosphatidylinositol (PI) fraction remained unchanged throughout the trial period. We conclude that in humans omega 3 fatty acids are incorporated into platelet membrane phospholipid subclasses with a high degree of specificity.