Abstract: The purpose of the investigation was to determine the effects ofeicosapentaenoic acid (EPA), docosahexaenoic acid (DHA)and arachidonic acid (AA)on the isolated rabbit aortic strips. The results showed that (1) the rabbit aortic strips withintact endothelium contracted in a dose-dependent manner to exogenous AA betweenthe concentrations of 2.5 ×10^-6 mol/L and 4 ×10^-5mol/L. EPA did not affect the tensionof the relaxing strips, but inhibited the contractile response to AA (2×10^-5mol/L), IC₅₀=9.94×10^-9 mol/L. DHA showed no significant effect on the contractile response to AA.(2) In the endothelium- damaged rabbit aorta, the contraction of strips to AA was drasti-cally diminished. The inhibitory effect of EPA on the contraction to AA was almost van-ished. (3)The contraction of rabbit aortic strips to AA was shown to be abolished byindomethacin ( 1.5 ×10^-5mol/L), a cyclooxygenase inhibitor .(4) Radioimmunoassayshowed that exogenous AA increased the tissue levels of 6- keto- PGF_1α, TXB₂ and theirratio. Lower dose of EPA( 1.5 ×10^-6 or 1.5 ×10^-5 mol/L)did not affect the tissue levels of6- keto - PGF_-α and TXB₂. However, EPA at higher concentrations decreased the meta-bolites of AA significantly. These findings suggest that exogenous AA induces the contraction of rabbit aortic strips in vitro,which is related to the endothelial cells and mediated by the metabolites(s) ofcyclooxygenase,likely endothelium- derived contracting factor(EDCF). EPA could inhibitthe contraction of rabbit aortic strips to AA,which depends upon endothelium and is possi-bly evoked through augmenting synthesis and release of endothelium- derived relaxing fac-tor(EDRF). DHA exhibited no influence on the AA induced vasocontraction.