BACKGROUND: L-arginine (L-Arg) is the precursor of endogenous nitric oxide (NO), which joins the pain regulation of peripheral myeloid level and above.OBJECTIVE: To study the role and mechanism of NO in nucleus raphe magnus (NRM) in pain regulation and electroacupuncture analgesia.DESIGN: A random control experiment on rats.SETTING: Physiology Department of Medical College of Xianning College.MATERIALS: The experiment was done during May 2002 to March 2003in the Physiology Department of Medical College of Xianning College. The 63 Wistar rats were randomized into 7 groups with 9 in each. ①. Five groups for the experiment of L-Arg dose-effect relationship: Normal saline group and L-Arg 1, 2, 4, 8 mmol groups. ②. Two groups for the experiment of relationship between L-Arg and electroacupuncture analgesia: Normal saline + electroacupuncture group and L-Arg + electroacupuncture group.METHODS: ①Experiment of L-Arg dose-effect relationship: Normal saline group was injected normal saline. L-Arg 1, 2, 4, 8 mmol groups were injected L-Arg 1, 2, 4, 8 mmol respectively. The volume was all 1.5 μL.Then, the hot water (50±0.5) ℃ was used to stimulate the tail once every 10 minutes to cause tail flick for testing the pain threshold. The observation was carried out continuously for 90 minutes. ②. Experiment of relationship between L-Arg and electroacupuncture analgesia: The two groups were injected normal saline and L-Arg 8 mmol respectively. The volume was all 1.5 μL. Ten minutes later, Zusanli (ST 36) of both posterior legs of rats were punctured with electric stimulation. The frequency was 4-16 Hz, and the intensity in an increasing order of 1, 2, 3V given 10 minutes of each. The electroacupuncture was totally 30 minutes, during which, testing the pain threshold 3 times. After needling, the pain threshold was still tested once every 10 minutes, till 90 minutes after injecting L-Arg.MAIN OUTCOME MEASURES: Pain threshold at different time points.RESULTS: The 63 rats entered the result analysis. ① The pain threshold of L-Arg 1 mmol group at all time points was lower than that of the normal saline group, but without statistical significance (P ＞ 0.05). The pain threshold of L-Arg 2, 4, 8 mmol groups was much lower than that of the normal saline group at all time points (P ＜ 0.001), and the effect was lasting to 80 minutes after injecting L-Arg, and with the increase of L-Arg density, the amplitude of pain threshold lowering was increased. ② The pain threshold of L-Arg + electroacupuncture group was much lower than that of the normal saline + electroacupuncture group at the time points of 20-50 minutes (P ＜ 0.01).CONCLUSION: ① The L-Arg injection to NRM can lower the pain threshold and there is a dose-dependent effect in it. ② The electroacupuncture can raise the pain threshold, and the L-Arg injection can weaken the effect of electroacupuncture analgesia. ③ NO in NRM joins in the pathological and physiological processes of pain and electroacupuncture analgesia. Its increase can lower the pain threshold greatly.