INDUCTION of long-term potentiation (LTP) in the hippocampus is associated with changes in expression of a variety of different proteins and is thought to be the mechanism which underlies synaptic plasticity. The 25 kDa synaptosomal-associated protein (SNAP-25) is a presynaptic protein which is involved in neurotransmitter exocytosis at the nerve terminal. Two isoforms of SNAP-25 have so far been identified (a and b) which differ in their distribution and developmental regulation. Usingin situhybridization, we demonstrated that the mRNA levels of the two isoforms of this protein are increased 2 h after the induction of LTP in granule cells of the dentate gyrus following high frequency stimulation of the perforant pathin vivo. These observations further demonstrate the involvement of both isoforms of SNAP-25 in functional synaptic plasticity, although their exact roles have yet to be fully determined.