Objective To probe into the prelude marker of central nervous system injury in response to methyl mercury chloride (MMC) stimulation and the signal transduction molecular mechanism of injury in rat brain induced by MMC. Methods The expression of c-fos mRNA in brain and the expression of c-FOS protein in cortex, hippocampus and ependyma were observed using reverse transcription polymerase chain reaction (RT-PCR) and immunocytochemical methods. The control group was injected with physiological saline of 0.9%, while the concentrations for the exposure groups were 0.05 and 0.5, 5 mg/kg MMC respectively, and the sampling times points were 20, 60, 240, 1440 min. Results The expression of c-FOS protein in cortex and hippocampus increased significantly, the accumulation of mercury in the brain induced by 0.05 mg/Kg MMC for 20 min had no significant difference compared with the control group. The mean value was 0.0044 mg/Kg, while the protein c-FOS expression had significant difference compared with the control group (P<0.01). More sensitive expression occurred in hippocampus and cortex, but not in ependyma. Conclusion The expression of c-FOS protein in cortex and hippocampus can predict the neurotoxicity of MMC in the early time, and immediately early gene (IEG) c-fos participates in the process of brain injury induced by MMC. Objective To probe into the prelude marker of central nervous system injury in response to methyl mercury chloride (MMC) stimulation and the signal transduction molecular mechanism of injury in rat brain induced by MMC. Methods The expression of c-fos mRNA in brain and the expression of c-FOS protein in cortex, hippocampus and ependyma were observed using reverse transcription polymerase chain reaction (RT-PCR) and immunocytochemical methods. The control group was injected with physiological saline of 0.9%, while the concentrations for the exposure groups were 0.05 and 0.5, 5 mg / kg MMC respectively, and the sampling times points were 20, 60, 240, 1440 min. Results The expression of c-FOS protein in cortex and hippocampus increased significantly, the accumulation of mercury in the brain induced by 0.05 mg / Kg MMC for 20 min had no significant difference compared with the control group. The mean value was 0.0044 mg / Kg, while the protein c-FOS expression had a significant difference compared to the control group with the control group (P <0.01). More sensitive expression occurred in hippocampus and cortex, but not in ependyma. Conclusion The expression of c-FOS protein in cortex and hippocampus can predict the neurotoxicity of MMC in the early time, and immediately early gene (IEG) c-fos participates in the process of brain injury induced by MMC.