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Objective:Central nervous system has unique inflammatory responses to the exposure to the endotoxin and immature brain may have a different response with that of the mature. The authors conduct this experiment to elucidate the characteristics of inflammatory response in immature brain. Methods:Lipopolysaccharide(LPS) was injected in the right caudate nucleus in 7-day-old and adult Sprague-Dawley rats . The doses were 1μl of 0.1, 0.5, and 2.0mg/ml of LPS and the same amount of saline for controls. The rats were sacrificed 24hours after injections. Light microscopic examination was performed to evaluate the leukocyte recruitment, and reverse transcriptase-polymerase chain reaction(RT-PCR) to measure the expression of interleukin-1β(IL-1β) and tumor necrosis factor-α(TNF-α) mRNA. Results:Light microscopic examination revealed more pronounced leukocyte infiltration in neonatal brain than in adult, even though lower than in peripheral tissue. RT-PCR revealed dose dependent expression of IL-1β and TNF-α mRNA in both neonatal and adult brain as in peripheral tissue. Conclusion:These results support that the immature brain is more vulnerable to the LPS induced inflammation in terms of leukocyte infiltration and possibly resultant brain damage. However, the mechanism of inflammatory response in immature brain should be studied further in association with the research of the activity of microglia, astrocyte, blood brain barrier, chemokine, and adhesion molecule in immature brain. Key words:Central nervous system;Inflammation;Lipopolysaccharide;Leukocyte;Tumor necrosis factor-α;Interleukin-1β.