Abstract： Objective:Clonidine,by activating peripheral α-sbrenoceptors, produces transient pressor response after i.v.injection in anesthetized animals.Moxonidine, with at least 40-fold higher affinity to I1-imidazoline receptors than to α2-adrenoceptors,produces also a transient pressor response. This work was designed to investigate whether I1-imidazoline receptors are involved in this pressor effect of moxonidine. Methods:Female spontaneously hypertensive rats(SHRs,aged 14-16 weeks)were anesthetized with urethane.To observe the transient pressor responses,moxonidine 0.1,0.3,1.0mg/kg(intravenous,i.v),2.0μg(intracerebroventricular,i.c.v.)and 1.0,10.0mg/kg(intragastric,i.g.)were administrated in different groups of rats.To evaluate the roles of α1-adrenoceptors,α2-adrenoceptors and I1-imidazoline receptors in the transient pressor responses to moxonidine, prazosin(10.0μg/kg),yohimbine(2.0mg/kg),phentolamine(0.2mg/kg),idazoxan(1.0mg/kg)or yohimbine+idazoxan(2.0mg/kg+1.0mg/kg)were intravenously given to the animals before moxonidine 0.3mg/kg (i.v.).Results:It was found that i.v.moxonidine produced a greater pressor response than clonidine when producing a similar reduction of blood pressure.This effect of moxonidine was not influenced by prazosin, but was partly inhibited by yohimbine, phentolamine or idazoxan,and completely blocked by the combination of yohimbine and idzaxon.Neither i.c.v.injection nor i.g. administration of moxonidine induced transient pressor responses.Conclusion:The transient pressor response of i.v. moxonidine is mediated by both peripheral I1-imidazoline receptors and α2-adrenoceptors.