In the present study, the effect of chronic oral administration of curcumin in the presence or absence of morphine and noloxone was investigated on the visceral nociception induced by acetic acid in rats. Intraperitoneal injection of acetic acid (1 mL, 2%) produced contractions in the abdominal musculature (writhes). The latency time to the beginning of the first writhe was measured and the total number of writhes in the 1 h after acetic acid injection was counted. The latency time to the beginning of the first writhe was significantly (p<0.05) increased and the number of writhes was significantly (p<0.05) decreased by curcumin (20 and 40 mg kg^-1 body weight). The same results were obtained after subcutaneous injection of morphine (1 mg kg^-1 b.wt.). Naloxone at the dose of 1 mg kg^-1 body weight had no effect on pain intensity. Curcumin significantly (p<0.05) enhanced the effect of morphine on the visceral pain responses, however did not reverse the effect of naloxone. Present data suggest that in the acetic acid-induced visceral nociception of rats, curcumin may produce an antinociceptive effect and the endogenous analgesic opioid system is involved in the curcumin-induced antinociception.