Validation of visceral pain induced with acetic acid. Visceral pain was induced with acetic acid in ICR species of mice. The onset (A) and the total number of writhes (B) during a period of 15 minutes were determined after s.c. administration of morphine (0.2, 0.6, 1.7, 5 mg/kg) 20 minutes before i.p. injection of 0.6% acetic acid. Intraperitoneal injection of 0.6% AA resulted in a significant reduction in onset time of writhes and increased number of writhes. Morphine significantly increased the onset and decreased the total number of writhes at a dose of 0.6-5 mg/kg (mean ± s.e.m., n = 6-10, *p < 0.05, **p < 0.01, ***p < 0.001).