Abstract: The experiments were performed on 4-30 days old rabbits of either sex anaesthetized with urethane 100 mg/100 g(I.p).The sympathetic nerves and depressor nerves of either necks were sectioned.The central end of vagus nerves (also scetioned) were stimulated via bipolar stainless steel electrodes.The blood presure in carotid artery was recorded by means of LMS-2A physiological recorder.The effects of intracerebroventricular injecting naloxone on the vagus pressor response were observed.The results showed that naloxone did not affect the pressor response in 4-15 days old rabbits (P>0.1);but it partially block the pressor value in rabbits of 20 days old or so (P>0.05);and the obvious blocking effects were observed in 25-30 days old rabbits (P<0.05,P<0.01).These suggested that the important time for the endogenous opiate like substance to be produced and its receptors system to develop was about 20 days after birth in rabbits and they began to modulate the vagus pressor response.