In this study,the average cortical evoked potential elicited by stimulating selectively tbe C-fibers (C-CEP) in the sapbenous nerve was taken as an index to reflect stow pain in order to observe the analgesic effect of He-Ne laser acupuncture. The results were as follows:1. Experiments carried out on 20 cats showed that both 1. 5 mW (irradiating distance 5 cm ,spot diameter 1. 5 mm,power density 84. 93 mW/cm2) and 15 mW(irradiating distance 10 cm ,spot diameter 2 mm,power density 477.71 mW/cm2) He-Ne laser irradiation on the median nerve inhibited very markedly the C-CEP.This suggested that laser acupuncture could inhibit slow pain.2. Intravenous injection of naloxone(0. 4 mg/kg) was able to reverse tbe inhibitory effect (on the C-CEP) caused by He-Ne laser acupuncture. This result indicated that the inhibitory effect of laser acupuncture analgesia was related to the activity of opiate-like substances(OLS).3. The induction period and tbe after effect of analgesia resembling those observed clinically in electro-acupuncture were also found in He-Ne laser acupuncture. In addition,in higher output power He-Ne laser (15 mW) irradiation in cats, two phases of inhibitory effect on the C-CEP were observed : the inhibition occurred firstly (1st phase) during and for a short duration immediately after tbe cessation of laser acupuncture, and then recurred (2nd phase) from the 3rd to the 20th min. after the cessation of laser Irradiation.4. From tbe results presented,it appeared that the effectiveness of He -Ne laser analgesia and tbe irradiated dosage of laser (i. e. tbe energy density denoted by j/ cm2) are,within certain limits, positively correlated.