Background & Aims: Although acupuncture has a significant clinical benefit, the mechanism of acupuncture remains unclear. Vasopressin, a posterior pituitary hormone, is involved in nausea and vomiting in humans and dogs. To investigate the anti-emetic effects of acupuncture on vasopressin-induced emesis, gastroduodenal motor activity and the frequency of retching and vomiting were simultaneously recorded in conscious dogs. Methods: In seven dogs, four force transducers were implanted on the serosal surfaces of the gastric body, antrum, pylorus, and duodenum. Gastroduodenal motility was continuously monitored throughout the experiment. Vasopressin was intravenously infused at a dose of 0.1U/kg/min for 20 minutes. Electroacupuncture (EA, 1-30 Hz) at pericardium-6 (PC6), bladder-21 (BL21) or stomach-36 (ST36) was performed before, during and after the vasopressin infusion. To investigate whether the opioid pathway is involved in EA-induced anti-emetic effects, naloxone (a central and peripheral opioid receptor antagonist) or naloxone methiodide (a peripheral opioid receptor antagonist) was administered before, during and after EA and vasopressin infusion. Results: Intravenous infusion of vasopressin induced retching and vomiting in all dogs tested. Retrograde peristaltic contractions (RPCs) occurred before the onset of retching and vomiting. EA (10 Hz) at PC6 significantly reduced the number of episodes of retching and vomiting. EA at PC6 also suppressed RPCs as well. In contrast, EA at BL21 or ST36 had no anti-emetic effects. The antiemetic effect of EA was abolished by the pretreatment with naloxone, but not naloxone methiodide. Conclusion: It is suggested that the anti-emetic effect of acupuncture is mediated via the central opioid pathway.