Pain Low power laser analgesic effect was generally accepted in clinical cases, whereas there was no direct evidence to indicate that low power laser irradiation suppressed an impulse conduction within a peripheral nerve. The effects of low power laser irradiation on electrically evoked responses within the sural nerve was electrophysiologically analyzed in unanesthetized, decerebrate cats and in anesthetized rabbits. Firstly, the effect of a low power (1mW; 632.5nm) helium-neon laser irradiation to the cord dorsum potentials evoked by the sural nerve stimulation was analyzed in unanesthetized, decerebrate cats. The cord dorsum potentials which were induced by the electrical nerve stimulation applied to a distal portion of exposed sural nerve was significantly suppressed during a low power laser irradiation. Secondary, the effects of low power laser irradiation upon multiple units' discharges within a peripheral nerve, which were induced by a noxious stimulation, were analyzed in anesthetized rabbits. Responding to a pinch stimulation of plantar skin, transitory increase of neuronal discharges, which reduced within a minute, and persistent increases, which continued during a period of the stimulation, were induced in the sural nerve. These increases became significantly smaller than the control value during low power laser irradiation that applied to the exposed sural nerve at distal to the recording site. Thirdly, the effects of low power laser irradiation on electrically evoked responses within the sural nerve were analyzed in anesthetized rabbits. Only high threshold evoked responses (unmyelinated A delta) which was induced by an electrical stimulation to the peripheral stamp of the nerve were significantly suppressed during low power laser irradiation which applied to the exposed sural nerve between the stimulus site and the recording site. The suppressive effect was reversible and recovered to the control level after the irradiation, Experimental evidences indicated that low power laser irradiation suppressed the impulse conduction of unmyelinated A delta afferents in peripheral sensory nerve which caused a pain sensation. Our data suggest that low power laser act as a reversible direct suppressor of neuronal activity.Wound healing Although many clinical studies have tested usefulness of low power laser (LPL) irradiation for the wound healing, the mechanism of the effects by LPL irradiation has not been fully elucidated. The present study demonstrated that the enhanced LPL irradiation effect to the procollagen production was consistently observed in the serum-starved medium, not in the FCS-contained medium. These results suggested that some factors in FCS might interfere with LPL irradiation onto procollagen synthesis. This view is supported by the fact that LPL irradiation was much more effective in intractable ulcers and bloodless wounds, which could be lack in FCS. The LPL irradiation effect was not blocked by dexamethasone but methylene blue. It seems probable that the LPL enhanced effects to the procollagen product of the fibroblast might be occurred at the translational level not at the pretranslational level of the mRNA synthesis and cGMP might be involved in the enhanced effect of the procollagen synthesis in fibroblast.In vivo study, the effect of the low power laser on the inflammatory stage of wound healing was investigated using delayed wound healing animal model. Round open wounds were made on the bilateral backs of doxorubicin treated pigs. After making wounds, low power laser was ipsilaterally irradiated on the wounds for 3 minutes everyday. On the 5th day, the wounds were removed and immunohistologocally stained using anti-fibronectin antibody and anti-keratin antibody. Intensity of the fibronectin-like immunoreactivity, the length of new epithelium and the number of the inflammatory cells were measured and compared in the presence and absence of LPL irradiation by using computed analyzing method.