['To investigate whether the timing of intrathecal administration of the opioid analgesic fentanyl, alters noxious stimulus-evoked neuronal activity in the rat spinal cord.', 'A 5% formalin solution was used as the noxious stimulant. For the pretreatment group, a dose of 0.001 to 0.5 microg of fentanyl was injected intrathecally ten minutes prior to formalin injection. Early and late post-treatment groups received 0.01 to 0.5 microg fentanyl, five and 60 min after formalin injection respectively. The effect of fentanyl was confirmed with naloxone. The level of c-Fos expression was determined in each treatment group to indicate neuronal activity.', 'Pretreatment and early post-treatment groups showed suppression of c-Fos activity compared to the vehicle (P <0.01). The late post-treatment group showed no difference in c-Fos activity compared to the vehicle (P=NS). Pretreatment with fentanyl showed the most profound suppression of c-Fos expression (P <0.01). In addition, pretreatment injection showed a greater suppression of c-Fos activity in the deep (14.6% of control) compared to the superficial laminae (32.7% of control; P <0.01), whereas the early post-treatment group showed a universal decrease in c-Fos activity (49.2% of control in laminae I and II, 50.4% of control in laminae III and IV and 51.8% of control in laminae V and VI). Naloxone reversed the action of fentanyl on c-Fos activity.'] Does intrathecal pre-administration of fentanyl effectively suppress formalin evoked c-Fos expression in spinal cord of rat?