Three different cell markers were studied in rats to note changes in the immunoreactivity (IR) in the lumbar spinal cord 1 to 84 days following partial sciatic nerve ligation (PSNL). Alteration in average IR were studied for complement receptor C3bi (OX42; microglia), major histocompatibility complex II (OX6; microglia), and glia fibrillary acidic protein (GFAP; astroglia). Thirty-four female rats underwent ligation of approximately 1/2 of the sciatic nerve (PSNL). This injury resulted in the development of mechanical allodynia, which was quantitated by the measurement of foot withdrawal threshold to the application of Von Frey filaments. Ipsilateral increase in IR of OX42 and GFAP was observed to occur within 2 days, maximized by day 14, and did not return to contralateral spinal gray matter IR levels by day 84 (time period of study). Increases in OX42 IR and GFAP IR were observed within the spinal segments innervated by the sciatic nerve. GFAP IR was not expressed in all lumbar segments. OX42 staining with the upper portion of the dorsal horn was found to localize within the areas of deafferentation demonstrated by loss of thiamine monophosphatase activity within the substantia gelatinosa. OX6 IR was only seen in a few cells within the ipsilateral gray matter, indicating that microglia did not become phagocytic. Both GFAP IR and OX42 IR were found to linearly correlate with allodynic behavior with OX42 IR being more statistically significant. Correlation of OX42 IR in only the upper portion of the dorsal horn (not including the neck) resulted in an even a greater level of significance. These findings demonstrate that microglia and astroglia are activated following PSNL and that their increase in IR correlates with the development of allodynic behavior.