This study was undertaken to define the mechanism of Siegesbeckiae Herba-induced relaxation in rabbit common carotid artery contracted by agonists. In order to investigate the effect of Siegesbeckiae Herba on contracted rabbit carotid arterial strips, transverse strips with intact or damaged endothelium were used for the experiment using organ bath. To analyze the mechanism of Siegesbeckiae Herba-induced relaxation, Siegesbeckiae Herba extract infused into contracted arterial strips induced by agonists after treatment of lanthanum chloride, indomethacin, atropine, $Nomega-nitro-{_L}-arginine$, cobalt chloride or methylene blue. The relaxation effect of Siegesbeckiae Herba was dependent on the presence of endothelium, showing that Siegesbeckiae Herba-induced relaxation was not observed in the strips without endothelium. The endothelium-dependent relaxation induced by Siegesbeckiae Herba was suppressed by the pretreatment of lanthanum chloride, $Nomega-nitro-{_L}-arginine$, cobalt chloride or methylene blue, but it was not observed in the strips pretreated with indomethacin or atropine. These results demonstrated that Siegesbeckiae Herba may inhibit agonist-induced contraction through an increase in the cyclic GMP by the production of nitric oxide in the vascular endothelial cells.