The opioid receptor mechanism involved in the morphine induced straub tail response was investigated in mice. Morphine (2.5, 5, 10 and 20 mg/kg s.c.) produced a dose dependent straub tail response and analgesia (hot plate test). Naloxone (5 mg/kg s.c.) and the mu-opioid receptor antagonist beta-funaltrexamine (10 micrograms i.c.v.) blocked both the straub tail response and analgesia while the mu 1-opioid receptor selective antagonist naloxonazine (35 mg/kg s.c.) blocked only analgesia and did not affect the straub tail response. Morphine (20 micrograms) administered by the i.c.v. route also produced the straub tail response as well as analgesia. Pretreatment with naloxonazine (35 mg/kg s.c.) antagonised i.c.v. administered morphine induced analgesia while the straub tail response was not affected. The results indicate that the morphine induced straub analgesia while the straub tail response was not affected. The results indicate that the morphine induced straub tail response is mediated by central mu 2-opioid receptors.