The present study was designed to investigate the effect of low power GaAsAl laser on Fos expression in the spinal cord induced by noxious mechanical stimulation. Noxious mechanical stimulation was applied to the right hind paw following 30min of low power laser treatment using different intensity and treatment point and the resulting Fos expression in the spinal cord dorsal horn was compared to that obtained in rats exposed only to the noxious mechanical stimulation. The results were summarized as follows: 1. In intact control rats, only a few Fos like immunoreactive(Fos-IR) neurons were evident in the lumbar spinal cord dorsal horn. Similarly, following prolonged inhalation anesthesia, Fos-IR neurons were absent in the dorsal horn of the lumbar spinal cord. In animals treated with noxious mechanical stimulation, neurons with nuclei exhibiting Fos immunostaining were distributied mainly in the medial half of ipsilateral laminae I-V at lumbar segments L3-5. These findings directly indicated that prolonged anesthesia used in this study did not affect the Fos expression in the spinal cord dorsal horn of intact animals and noxious mechanical stimulation treated animals. 2. In acupoint treated animals, 10mW of laser stimulation, not 3mW intensity, significantly reduced the number of Fos immunoreactive neurons in the spinal dorsal horn induced by noxious mechanical stimulation(P<.01). However, the supressive effect of low power laser stimulatin was not observed in 3m Wand 10m W of laser stimulation into non-acupoint. These data indicate that 10mW of low power laser stimulation into acupoint is capable of inhibiting the expression of Fos in the dorsal horn induced by noxious mechanical stimulation. In conclusion, these findings raise the possibility that low power laser stimulation into acupoint may be a promising alternative medicine therapy for the mechanical stimulation induced pain in the clinical field.