Objective To investigate the effect and possible mechanism of oxytocin on the expression of NO and ET-1 in precon-ditioning myocardium of rats with ischemia .Methods The rats were randomly divided into ischemic reperfusion group(I/R) ,ische-mic reperfusion + oxytocin pretreatment group(I/R+OT) ,sham operation group(SH) and nomal control group(N) .The myocar-dial ischemia-reperfusion model was induced by 25 min coronary occlusion and 120 min reperfusion in opened-chest anesthetized rats ,rats of SH group were not conducted with ligation but threading only .Group I/R+ OT were administered oxytocin(0 .03 g/kg ,i .p)25 min prior to ischemia ,the other three groups were given the same amount normal saline .After reperfusion ,nitric oxide (NO) ,endothelin-1(ET-1) ,creatin kinase isoenzyme(CK-MB)and lactate dehydrogenase(LDH)in blood serum were measured .At the end of reperfusion ,HE stain was used to observe the morphology of the cardiac muscular tissue .Results Pathological observa-tion showed that the myocardial cell in group SH and in X ranged in order ,with no fibers fractured ,myocardial fibers in I/R group were fractured and disorganized ,interstitial edema ,neutrophil infiltration were also observed .It was more serious in group I/R than in group I/R+OT .Compared with SH group ,the CK-MB ,LDH ,ET-1 of the I/R group and I/R+OT group increased ,the NO de-creased(P<0 .05) ,and it had no difference in group N and group SH (P>0 .05) Compared with the I/R group ,in I/R+OT group the expression of CK-MB ,LDH ,ET-1 were decreased ,the level of NO increased(P<0 .05) .There was a negative correlation be-tween NO and ET-1 in group I/R and I/R+ OT (P< 0 .05) .Conclusion Oxytocin pretreatment on rat myocardial ischemia and reperfusion has protective effect ,and its action mechanisms maybe associated with the increase of the content of NO and the de-crease of the content of ET-1 in blood serum .