The effects of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on the arachidonic acid(AA) metabolism were examined by incubating separately washed human platelets and human umbilical artery rings with labelled AA. The results are as follows:1.Both EPA and DHA reduced the formations of TXB2 in platelets and 6-keto-PGF1a in artery walls in dose-dependent way.2.The effect of EPA on the synthesis of HETE by platelets was different with the concentrations of EPA. At lower concentration (62.5?M), it increased the formation of HETE, but the degree of increase was lowerd when the concentration of EPA was increased further, and even reduced the formation of HETE when its concentration reached 1000?M although the decrease extent was statistically insignificant.3.At all of concentrations used but 62.5?M, DHA raised potently the formation of HETE by platelets in dose-dependent way.A brief discussion was made about our results with special references to former data reported.