subjective: The effect of 4~5 Hz electroacupuncture (EA) on alterations of both met enkephalin (MEK) and dynorphin (Dyn) in the patient plasma or mouse spinal cord and its relation with analgesic effect were studied.Methods: In acupuncture clinic 10 patients with acute pain were treated with 4 Hz EA at Zusanli(ST 36) and/or Hegu(LI 4) acupoints for 30 min. 20 BALB/C mice were randomly divided into 2 groups: a. EA group(n=10), treated with 4~5 Hz EA at bilateral “Zusanli”(ST 36) for 15 min; b. control group(n=10) treated with no EA, but also restrained for 15 min. Before and after EA or restraining acupoints, the pain threshold of the patients or mice was detected. 10 μl of the patient plasma before and after EA and each mouse spinal cord suspension, of the 2 groups were blotted onto nitrocellulose membrane (NCM) respectively. The protein dot blot signals were detected by immunoreactivity (IR) and using Shimadu TLC Scanner and analyzed statistically. Results: The results showed that an increase in patient plasma MEK IR or Dyn IR and a decrease in mouse spinal MEK IR or Dyn IR could be detected, and the alteration of plasma or spinal MEK IR was more significant than that of plasma or spinal Dyn IR. There was a positive correlation in alteration between plasma or spinal MEK IR and plasma or spinal Dyn IR with respective parallel levels in individuals. The increased plasma MEK IR or the decreased spinal MEK IR was positively or negatively correlated with the analgesic effect, while the correlation between plasma or spinal Dyn IR and analgesic effect was insignificant. Conclusion: The results suggest that under lower frequency EA the met enkephalin may play an important role in analgesia.