PURPOSE: The pathways of pain conduction in brain are not well known. Also, differences of somatic pain conduction between adult and young age have not been fully elucidated. This study was conducted to investigate any differences in the expression of c-Fos protein between adult and young rats after somatic pain was induced by formalin. METHODS: Male rats (n=70) were injected subcutaneously with 0.1mL of 5% formalin in the plantar surface of right hindpaw. Rats were sacrificed at 30 minutes, 1 hour, 2 hours, 6 hours, 24 hours after noxious formalin stimuli to hindpaws and rectums. Rat brains were removed and sliced in rat brain matrix. Brain slices were coronally sectioned at interaural 5.70-6.70mm. Serial sections were immunohistochemically reacted with polyclonal c-Fos antibody. The numbers of c-Fos protein immunoreactive neurons in cingulate cortex, primary somatosensory area, and hippocampus were examined and analyzed statistically with Mann-Whitney U test. RESULTS: The number of c-Fos protein immunoreactive neurons in the cingulate cortex, primary somatosensory area and hippocampus peaked at 2 hours after formalin-induced pain on adult rats. The number of c-Fos protein immunoreactive neurons in the cingulate cortex, primary somatosensory area and hippocampus peaked at 1 hour after formalin-induced pain on young rats. The numbers of c-Fos protein immunoreactive neurons of adult groups were higher than that of young groups at all points of time. CONCLUSION: The immunoreactions in adult group expressed more intense than those in young group. Earlier expression of immunoreactions in young group suggests of faster conduction of pain, compared to those in adult group. Larger number of c-Fos protein immunoreactive neurons were found within specific regions in both groups. These results could provide some basic knowledge in understanding the mechanism and control of pain in pediatric group.