Neuronal responses in thalamic nucleus parafasciculus (Pf) to microiontophoretic rnorphine wereexamined in 20 wistar rats after lesion of SmI. Extracellular recordings were rnade with five-barreledglass microelectrodes. The results showed that the spontaneous responses of Pf neurons were not statistically changed (n= 11

P>0.05)

but their nociceptive responses were attenuated at 0- 8 min utes after iontophoretic morphine (n=11

P<0. 05 ). There were statistically significant differencesin nociceptive responses between morphine and saline groups at 0-8 minutes after iontophoresis. Theresults suggest that morphine may be involved in corticofugal modulation of nociceptive responses inthalamic Pf neurons.