Abstract: Objective To study the effect of oxytocin（OT） on vascular dilation and its mechanism. Methods Changes in vascular ring tension were observed with an in vitro vascular function experimental device.Blood vessels were taken from 6 rats of each group with their endothelium removed or preserved.OT dilating curves were plotted after the blood vessels were pretreated for 30min. Results OT could not dilate the vascular rings in endothelium of pregnant rats pretreated with phenylephrine（PE）.The OT（0.1U-3.2U） dilated the thoracic aorta ring of rats with their endothelium preserved or removed in a PE concentration-dependent manner.Nitro-L-arginine methyl ester（L-NAME）,a nitric oxide synthase inhibitor,showed an inhibitory effect on vascular dilation induced by PE（P＜0.01）.Indomethacin（Indo）,a cyclooxygenase inhibitor,had no effect on vascular dilation induced by PE.Pretreatment with 3mmol/L of non-selective K⁺ channel blocker（TEA） could decrease the effect of OT on vascular dilation.The effect of OT on vascular dilation induced by PE was not related with the Ca²⁺ concentration. Conclusion OT can dilate the blood vessels in rats.Its mechanism is related with NO-cGMP and potassium channels in smooth muscle of blood vessels.