Abstract： The effect of agmatine (Agm) on vascular tension and the underlying receptor mechanism were investigated in the isolated aortic artery of rats. The results are as follows. (1) Agm (10－7～10－2mol/L) relaxed aortic rings in a concentration-dependent manner under the condition of precontraction induced by phenylephrine (PE) at a concentration of 10－6 mol/L. (2) Either in the intact or the endothelium-denuded rings, pretreatment with NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 0.5 mmol/L) did not affect the vascular relaxant action of Agm, implying that the concentration-dependent vasorelaxation caused by Agm is not endothelium-dependent and NO is not involved. (3) Agm also relaxed aortic rings in a concentration-dependent manner under the condition of precontraction induced by CaCl2 at a concentration of 3 mmol/L. (4) Idazoxan (10－4 mol/L), an α2-adrenergic receptor (α2-AR) and imidazoline receptor (IR) antagonist, abolished the Agm-induced vasorelaxation completely under the condition of CaCl2-induced precontraction. (5) Yohimbine (10－4 mol/L), a selective α2-AR antagonist, could partially block the vascular relaxant action of Agm. It is suggested that the vascular relaxant effect of Agm on the rat aortic artery may be mediated by α2-AR and IR.