Introduction: In the present study we attempted to investigate the alterations of cytological parameters of NRM (Longaxis and intranuclear rods) in the absence of its excitatory VL PAG afferent with or without the formalin pain model. Materials and Methods: We used male rats in 4 groups: 1- control group (n=4), 2- formalin group (n=4), 3- chemical lesion of VL PAG by Ibotenic Acid (0.2 َl, 0.5 َL) without formalin test (n-6), 4- chemical lesion of VL PAG by Ibotenic Acid (0.2َL) with formalin test. one week after formalin test or chemical lesion of VL PAG, animals were perfused and 3-5 mm blocks of their brainstem were stained by Thionin. Serial sections (5-8 micrometer) were examined by light microscope. Results: Our results revealed that three types of neurons were observed within NRM: fusiform, triangle and multipolar. In addition, following chemical lesioning of VL PAG or formalin induced pain, a marked alteration in some cytological characteristics of these neurons has been observed. Conclusion: We conclude that the VL PAG-NRM pathway is involved in the tonic pain nociceptive control system. Moreover, NRM neurons could react to noxious stimuli with cytological changes.