Previous findings have indicated that long-term potentiation (LTP) in the perforant path-granule cell synapses is accompanied by an increase in K+-stimulated, calcium-dependent release of [3H]glutamate 45 min after the induction of LTP. Here we report that release of [3H]glutamate is increased at 3 time intervals following induction of LTP, 2.5 min, 45 min and 3 h. Stimulation of the commissural input to the granule cells blocks the induction of LTP and the increase in [3H]glutamate release.