Objective To study c- fos and substance P expression in the central nervous system following mechanical and chemical nociceptive stimulation to the masseters in rats with occlusal interference. Methods Occlusal interference was made by bonding a 2 mm long dentin screw in the pulp cavity of the first maxillary molar in the left side. Seven days after occlusal interference, the rats in occlusal interference and mechanical stimulus group and mechanical stimulus control group were light anesthetized and nociceptive mechanical stimulus were applied to the ipsilateral masseter. Pain response was recorded and all the animals were killed 2 hours later. The rats in the other two groups were deep anesthetized and 100 μL 5% formalin was injected into the ipsilateral masseter, killed 2 hours later. The brainstem and cervical spinal cord were processed c- fos and substance P immunoreactivity and data were quantitatively analyzed. Results Both mechanical and chemical stimulus to the ipsilateral masseter induced increasing neuronal c- fos expression in the trigeminal nucleus and in the cervical spinal dorsal horn in occlusal interference and mechanical stimulus group and occlusal interference and chemical stimulus grou（p P
