The effect of opioid peptides on corticofugal modulation of nociceptive response in thalamic ventrobasal nucleus (VB) produced by cortical sensorimotor area Ⅱ (Sm Ⅱ) was observed.Single unit activities of VB neurons were extracellularly recorded by glass microelectrodes.The results showed that the nociceptive responses of VB neurons were attenuated at 0- 7. 5 minutes after stimulating Sm Ⅱ(n=14

P<0.05). The inhibitory effect of stimulation of Sm Ⅱ was reduced after ventriculer microinjection of 20ul of naloxone (n=9)

while marked inhibition was shown at 0- 2. 5 minutes after stimulating Sm Ⅱ in the saline control group (n= 9

P<0.05). There was a statistical difference at 0 - 2. 5 minutes after stimulating Sm Ⅱ between the two groups (P<0.05). These studies suggest that opioid peptides might be involved in corticofugal modulatio of pain