The effect of a diet rich in eicosapentaenoic acid (EPA) on platelet phospholipid fatty acid composition, platelet aggregation, and bleeding time was studied in 10 healthy men, whose usual diet was partly replaced by fish for 11 weeks. This diet provided 2-3 g EPA per day. Two doses (3.5 and 10 mg/kg body-weight) of acetylsalicylic acid (ASA) were given before and during the diet. The fish diet prolonged bleeding time (by 42%) and decreased platelet aggregability. The changes in platelet phospholipid fatty acid composition consisted of increases in the omega-3 series (C20:5 and C22:6) and decreases in the omega-6 series (C18:2 and C20:3). The reduction in platelet aggregation induced by collagen and ADP did not parallel the changes in platelet membrane phospholipids and bleeding times. Diminished platelet aggregation induced by collagen lasted only 3 weeks (while subject was still on the diet), whereas the decreased sensitivity to ADP persisted for at least 11 weeks after the volunteers had resumed their normal diet. ASA taken before the diet prolonged bleeding time by as much as did the diet itself. ASA taken during the diet prolonged bleeding time by more than the sum of the increases in bleeding time caused by ASA and by the EPA diet separately, but the synergism was not significantly more than additive. The findings suggest that a diet rich in omega-3 polyunsaturated fatty acids reduces tha interaction between platelets and the vessel wall by mechanisms which are more complex than just a reduction in susceptibility of platelets to the naturally occurring agents collagen and ADP, or an imbalance between proaggregatory and anti-aggregatory prostaglandin derivatives.