OBJECTIVE To study the changes of interleukin-1 beta (IL-1beta), tumor necrosis factor alpha (TNFalpha) and interleukin-6 (IL-6) levels in brain and plasma after brain injury and to assess the relationship between the cytokine levels and injury severity in rats. METHODS A total of 51 male Wistar rats, weighing 280-340 g, were anesthetized with chloral hydrate (400 mg/kg body weight) through intraperitoneal injection and fixed on a stereotaxic instrument. Severe brain injury was created in 16 rats (severe injury group) and moderate brain injury in 18 rats (moderate injury group) by a fluid percussion model, and cytokine levels of IL-1beta, TNFalpha and IL-6 were measured with biological assay. And sham operation was made on the other 17 rats (control group). RESULTS In the control group, the levels of IL-1beta, TNFalpha and IL-6 were hardly detected in the cortex of the rats, but in the ipsilateral cortex of the rats in both injury groups, they increased obviously at 8 hours after injury. The increasing degree of these cytokines had no significant difference between the two injury groups. The levels of IL-6 in the plasma of all the rats increased slightly, whereas the levels of IL-1beta and TNFalpha were undetectable. CONCLUSIONS The increase of IL-1beta, TNFalpha and IL-6 levels is closely related to brain injury. The increased cytokine levels in the central nervous system are not parallel to those in the peripheral blood. It suggests that inflammatory cytokines play important roles in the secondary neural damage after brain injury.