The microinjection of brain nuclei and potassium iontophresis induced tail flick were used to investigate the effect of injecting oxytocin (OT) or anti-oxytocin serum (AOTS) into locus ceruleus (LC) on the pain threshold (PT) and morphine analgesia. The result showed that OT injection into LC could enhance PT, while ATOS injection could reduce PT. The OT injection could strengthen morphine analgesia, but AOTS injection could antagonize this action. These results suggest that the OT analgesia and OT-enhanced morphine analgesia were related with the locus ceruleus.