Objective: To study vasodilation effect of a water decoction of the Common Monkshood Prepared Daughter Root(DCMR) on rabbit
thoracic aortic rings and its mechanism. Method: In noradrenaline(NA) precontracted isolated aortic rings in rabbits, the response to different
concentration of DCMR was investigated; the effects of removal of endothelium, coincubated with nitric oxide synthase inhibitor L NNA, methylene blue(MB), prostaglandin synthase inhibitor and blockage of the adrenergic β receptor on DCMR induced relaxation were also assessed. Result: DCMR did not change the resting tension of rabbit aortic rings but DCMR with different concentration could cause an obvious relaxation in 10 6 mol/L NA precontracted aortic rings(r=0.87,p<0.05).The relaxant effect of DCMR was significantly reduced by removal of endothelium 10 4 mol/L L NNA and 10 5mol/L MB, but not by prostaglandin synthase inhibitor and blockage of the adrenergic β receptor (Prop). In addition DCMR(1mg/mL)did not inhibit NA and KCl cumulative concentration response curves of aortic rings without endothelium, and did not change the PD 2 values for NA from(7.07±0.90)in control group to (6.93±0.87,P>0.05)and for KCl from(1.40±0.36) in control group (1.25±0.21) (P>0.05) respectively. Conclusion: DCMR can relax isolated rabbit aorta, which may be endothelium dependent and involve in nitric oxide (NO), but is not related to blockage of receptor operated and voltage dependent calcium channels.