We examined whether long-term potentiation (LTP) affects cortical gamma-band electroencephalograms (EEG) in the hippocampo-prefrontal cortex (PFC) pathway of anesthetized rats. The LTP induction increased the evoked PFC gamma-band EEG power (40-100 Hz) to 120-135% at 500-700 ms after test stimulation. A simple increment of stimulus intensity, instead of LTP induction, did not reveal this evoked increase. Neither LTP induction nor the intensity increment changed significantly the magnitude of an evoked decrease at around 100 ms or the spontaneous prestimulation gamma-band power. These results indicate that LTP in PFC specifically increases the evoked gamma-band EEG power, which may reflect a phasic mode of plastic neurotransmission through the hippocampo-PFC pathway in vivo.