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Background: The antinociceptive effect of an aqueous extract from the leaves of Toona sinensis (TS, [A. Juss., M. Roem.]) was studied using the writhing test in mice. Methods: Different extraction fractions from TS leaf extracts (TSL1 to TSL5) were administered orally 1 h before intraperitoneal injection of acetic acid. Results: After treatment with TSL1, TSL2, TSL3, TSL4, and TSL5 at a dose of 1 g/kg, the respective writhing responses were 39.9% (P < 0.001), 19.9% (P < 0.05), 11.7% (P = 0.052), 8.1% (P = 0.188), and 11.4% (P = 0.057) lower than the control group. Mice treated with TSL1 at 1 g/kg (39.9%, P < 0.001), 0.3 g/kg (38.0%, P < 0.001), 0.1 g/kg (46.9%, P < 0.001), and 0.03 g/kg (31.1%, P < 0.001) had significantly lower writhing responses compared with control mice. A time-course experiment was performed, which involved oral administration of TSL1 (0.1 g/kg) at 0, 0.5, 1, 2, and 6 h before acetic acid intraperitoneal injection. The most effective dose of TSL1 was 0.1 g/kg orally, with the effect beginning 30 min before treatment and persisting until 6 h. Conclusions: This study showed that TS has anti-visceral pain properties comparable with those of rofecoxib (a cyclooxygenase-2 inhibitor) and diclofenac, which suggests promise for the treatment of intractable visceral pain in humans. [ABSTRACT FROM AUTHOR]