Thirty ml of an emulsion containing 3 g of trieicosapentaenoylglycerol (90 % pure, containing 5 % arachidonic acid (AA)) was infused intravenously in 2 male healthy volunteers. Urine samples were collected for 24 h before and 48 h after the infusion in 5 periods. Urinary metabolites of prostaglandin (PG) I 2/3 and thromboxane (TX) A 2/3 (PGI 2/3-M and TXB 2/3-M, respectively) were extracted from the urinary samples and measured by GC-MS. Excretion of PGI 3-M was markedly enhanced right after the infusion. Because PGI 3 was produced without involvement of intestinal absorption of eicosapentaenoic acid (EPA), enhanced PGI 3 formation was strongly suggested to take place in the vasculature. From the marked increment in TXB 2/3-M after the infusion it was calculated that conversion rate of EPA to TXA 3 was 8 % of that of AA to TXA 2 in this in vivo condition.