of administration [intraperitoneal, oral, acute and chronic] . Sixty adult female rats were divided into six groups. They received saline, morphine or tramadol [20 to 125 mg/Kg] daily for 15 days. A hot plate test was performed for the rats at the 1 [st] , 8 [th] and 15 [th] days. After drug withdrawal, the hot plate test was repeated at the 17 [th] , 19 [th] , and 22 [nd] days. There was a significant correlation between the day, drug, group, and their interaction [P<0.001] . At 1 [st] day [d1] , both morphine, and tramadol caused an increase in the hot plate time comparing to the saline groups [P<0.001] , while there was no correlation between drug administration methods of morphine and/or tramadol. At the 8 [th] day [d8] , morphine and tramadol led to the most powerful analgesic effect comparing to the other experimental days [P<0.001] . At the 15 [th] day [d15] , their effects diminished comparing to the d8. After drug withdrawal, analgesic effect of morphine, and tramadol disappeared. It can be concluded that the analgesic effect of morphine and tramadol increases with the repeated use of them. Thereafter, it may gradually decrease and reach to a level compatible to d1. The present data also indicated that although the analgesic effect of morphine and tramadol is dose-and-time dependent, but chronic exposure to them may not lead to altered nociceptive responses later in life