This experiment was designed to study the expression of c-fos mRNA in brain following moderate lateral fluid percussion brain injury in rats and to observe the temporal pattern of its expressions following percussion. Male Sprague-Dawley rats were divided into normal control, sham operation control and injury groups. The rats of the injury group were subjected to moderate lateral fluid percussion injury (0.2 MPa). The injury group was then subdivided into 5 min, 15 min, 30 min, 1 h, 2 h groups according to the time elapsed after injury. The expression of c-fos mRNA was studied with reverse transcription polymerase chain reaction(RT-PCR) semi-quantitatively. c-fos mRNA in cortex and brain stem was expressed weakly in control groups. After 5 min of percussion, the expression of c-fos mRNA increased progressively and remained elevated up to 2 h after brain injury. This result suggested that the induction and expression of the c-fos mRNA in cortex and brain stem after fluid percussion brain injury were increased rapidly. The temporal pattern of induction in cortex was similar to that in brain stem.