OBJECTIVE To observe the effect of electroacupuncture (EA) on bispectral index (BIS) and plasma beta-endorphin (beta-EP) level in patients undergoing colonoscopy. METHODS Sixty patients were equally randomized into EA group and control group with 30 cases in each. EA (2 Hz/100 Hz, 4-6 V) was applied to the right Zusanli (ST 36) and Shangjuxu (ST 37), and the left Yinlingquan (SP 9), Sanyinjiao (SP 6) and bilateral Hegu (LI 4) respectively 30 min before colonoscopy. The mean arterial pressure (MAP), heart rate (HR) and BIS in two groups were continuously monitored during the study. Plasma beta-EP concentration was detected by radioimmunoassay. The patient's adverse reactions (including pain, satisfaction degree, etc.) were evaluated by visual analog scale (VAS) and verbal stress scale (VSS). RESULTS Self-comparison showed that MAP and HR in control group increased significantly during colonoscope's splenic flexure passing (P<0.05). Whereas the 2 indexes in EA group had no significant changes during colonoscope insertion, and its splenic flexure passing, hepatic flexure passing and post-enteroscopy (P>0.05). Comparison between two groups showed that MAP at the time-point of colonoscope insertion, and HR at the time-point of colonoscope's splenic flexure passing in EA group were significantly lower than those in control group (P<0.05). BIS values of EA group were significantly lower than those of control group at different time-points after colonoscope insertion (P<0.01). Plasma beta-EP concentrations at the time-points of colonoscope's hepatic flexure passing and post-enteroscopy were evidently increased in both groups in comparison with pre-enteroscopy (P<0.01), and beta-EP was significantly lower in EA group than that in control group at the time-point of colonoscope's hepatic flexure passing (P<0.05). The dosage of Midazolam used for conscious-sedation and the scores of VAS and VSS were also considerably lower in EA group than those in control group (P<0.05, P<0.01). No significant differences were found between two groups in the adverse reactions as dizziness, nausea, vomiting and abdominal pain, but the patients' satisfaction degree in EA group was evidently higher than that in control group (P<0.05). CONCLUSIONS Acupuncture analgesia can effectively lower the colonoscopy patients' BIS value and plasma beta-EP level, meaning attenuation of the patients' stress responses during colonoscopy after EA.