The effect of a combination of aspirin and fish oil on eicosanoids was studied. Four subjects were given 37.5 mg aspirin orally, and 6 weeks later they received a natural, stable fish oil daily for 1 week before taking the same single dose of aspirin. Blood samples for determination of whole blood production of eicosanoids were taken before and after each experimental period. Serum thromboxane A 2 was decreased by 40% (P<0.05) after aspirin alone, but by 62% (P<0.01) after fish oil + aspirin. Serum prostacyclin (measured as 6-keto PGF 1a ) was decreased by aspirin in both cases. The sum of 6-keto PGF 1a and its equipotent fish oil-derived analogue Δ 17 -6-keto PGF 1a was reduced after aspirin intake (55%, NS), but after fish oil + aspirin the reduction was smaller (33%, NS). Leukotriene B 4 was increased by 19% (P<0.05) after aspirin, and decreased by 69% (P<0.05) after fish oil + aspirin. A combination of stable fish oil and aspirin thus improves the eicosanoid pattern more than aspirin alone.