Abstract: AIM To observe the effects of intracerebroventricular administration of endomorphin-1 on blood pressure in anesthetized rats and to assess its mechanism. METHODS Variations of mean arterial pressure (MAP) were observed after icv injection of endomorphine-1 in rats. The effects of iv or icv of various blockers, naloxone, phentolamine, propranolol, atropine and N^ω-nitro-L-arginine, on the variation of MAP caused by endomorphin-1 were observed. RESULTS Endomorphin-1 was shown to decrease MAP. The decrease in MAP was blocked by naloxone. Pretreatments with icv phentolamine and propranolol showed no effect on the vasodepression induced by icv endomorphin-1. However, pretreatment with icv atropine (25 μg·kg^-1) attenuated the vasodepression. Pretreatments with iv phentolamine, propranolol and N^ω-nitro-L-arginine showed no effect on the vasodepression induced by icv endomorphin-1. But, pretreatment with iv atropine (50 μg·kg^-1) and bilateral vagotomy attenuated the vasodepression. CONCLUSION Intracerebroventricular administration of endomorphin-1 produces vasodepressor response in anesthetized rats, which is mediated by opioid receptor. The vasodepression is associated with the central M cholinoceptor and the excitation of the vagus.