When the entorhinal cortex is electrically stimulated, a monosynaptic evoked potential is produced in the ipsilateral hippocampus. Twenty-four hours after a 120-pulse stimulation train, the response to a single test pulse was found to be increased. Stimulation trains presented at a rate of 1/day had a cumulative effect on both the population EPSP and spike components of the evoked potential, and the potentiation so produced lasted at least 12 days. In a few cases, retested 2 months after the last stimulation train, the population EPSP was found to be still highly potentiated while the population spike had declined to initial levels. The most reliable results and largest potentiation were obtained with diphasic stimulation trains of 10 Hz or greater, though some changes were observed even with very low frequencies (0.2 Hz). The results imply that the efficiency of synaptic transmission in the perforant path-granule cell synapse can be modified.