The clinical usefulness of morphine for treatment of chronic pain is limited by development of tolerance. So many authors have proposed the co-administration of other drugs. This study was performed to evaluate the role of gabapentin on analgesic effect of morphine. Four groups of male rats (220-250 g) were examined for time latency using tail flick test: control, morphine (M), gabapentin (GB), and gabapentin-morphine (GB-M)-treated groups. Rats received morphine (10 mg/kg, s.c.) or gabapentin (75 mg/kg, i.p.) or both of them twice a day for 10 days. Control rats received normal saline as scheduled time. Tail flick latency time was recorded 3 times (5 minute interval) before drug injection and at 60, 65 and 70 minutes after drug injection. In this respect, %MPE was calculated for all groups. Analysis of variance showed no significant difference of %MPE in control and GB groups, while in M and GB-M groups, %MPE changed significantly during the study. GB had no analgesic effect, while M and GB-M had significant analgesic effect as compared to control. %MPE of GB-M was also significantly higher than M at days 5, 7 and 9. It can be concluded that co-administration of gabapentin enhances the analgesic effect of chronic use of morphine. Precise mechanism(s) should be investigated in future.