Objective To investigate the difference of Brn-4 mRNA expression between the fimbria/fornixtransected and normal side hippocampi. Methods Six SD rats’ right fimbrias were transected. On the 14th day after transection, the cryostat sections of hippocampus were prepared. Digoxigeninlabeled Brn-4 RNA probe was prepared and used in hybridization. Three sections were randomly taken from each rat, the number and the mean absorbance value of Brn-4 mRNA positive cells was measured. Paired t test analysis was used in statistic analysis. Results Brn-4 mRNA was expressed in pyramidal layer of hippocampus and granular layer of dentate gyrus in both normal and transected sides. The number of Brn-4 mRNA positive cells had no difference between two sides, but the mean absorbance value of positive cells in transected side was higher than that in normal side (P<0.01). In hilus and subgranular layer of dentate gyrus, not only the number of Brn-4 mRNA positive cells but also the mean absorbance value in transected side was higher than that in normal side (P<0.01).Conclusion In pyramidal layer of hippocampus and granular layer of dentate gyrus, Brn-4 mRNA expression is higher in fimbria/fornixtransected side than those in normal side. In hilus and subgranular layer of dentate gyrus, not only the number of positive cells but also the expression level of Brn-4 mRNA in transected side is higher than that in normal side. With our previous detection, the results suggest that the process in which the expression of Brn-4 mRNA increases after fimbria/fornix transection might be related to th