We have investigated the expression of the mRNAs for brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4) in the hippocampus before and after induction of long term potentiation (LTP) of synaptic transmission in the dentate gyrus through stimulation of the perforant path (PP). A unilateral PP stimulation produced a bilateral increase in the mRNA for both BDNF and NGF in granular neurones of the dentate gyrus but not in other neurones in the hippocampus. The mRNA for neurotrophin-3 (NT-3) was bilaterally decreased by LTP but that of NT-4 remained at the basal level. These results suggest that individual neurotrophic factors may play different roles in neuronal plasticity.