BACKGROUND Injection of formalin in the hindpaw of the rat induces intense C-fiber activity accompanied by brief flinching of the injected paw (phase 1) and gives rise to facilitated spinal processing characterized by renewed flinching beginning 15 min after injury and lasting 40 min or more (phase 2). In previous work, isoflurane, administered during phase 1, slightly reduced phase-2 activity, whereas the addition of intrathecal morphine dramatically inhibited phase 2, even with naloxone reversal 6 min after the formalin injection. We used a similar model to determine whether intrathecal morphine could block spinal sensitization in the absence of inhalation anesthetic. METHODS Hot plate tests at 52 degrees C and radiant heat-evoked hindpaw withdrawal tests were used to determine optimal doses of agonists and antagonists. The formalin test was carried out on male Sprague-Dawley rats, which were divided into five groups. A combination of naloxone 0.5 mg/kg and naltrexone 0.5 mg/kg was administered subcutaneously 6 min after the formalin injection to all animals except controls (group 1) to prevent ongoing opioid effect. Groups 1-3 received intrathecal saline, and groups 4 and 5 received intrathecal morphine 30 micrograms 20 min before formalin injection. Halothane was administered for 1-2 min to facilitate formalin injection for groups 1, 2, and 4. In groups 3 and 5 halothane was administered from 5 min before to 6 min after formalin injection. The number of flinches per minute was counted 1 and 5 min after formalin administration and thereafter at 5-min intervals for 1 h. The total number of flinches at 1 and 5 min was considered as phase-1 activity, and the total number of flinches during the 10-60-min interval was considered as phase 2. RESULTS Phase-2 activity for groups 1 and 2 was nearly identical, demonstrating no appreciable effect of the opioid antagonists alone. Groups 3 (halothane alone) and 4 (morphine alone) exhibited a significant decrease in phase-2 activity. Group 5 (morphine plus halothane) demonstrated a profound decrease in phase-2 activity, which was significantly more profound than that of groups 3 or 4. CONCLUSIONS Intrathecal morphine, administered before formalin injection but antagonized before the onset of phase 2 of the formalin test, significantly suppresses sensitization of dorsal horn neurons. This suppression is significantly increased by coadministration of halothane anesthesia.