This study examines gamma (40–100 Hz) oscillations in the hippocampus of awake rats. Gamma waves were highly coherent along the long axis of the dentate hilus but decreased in coherence in the CA3 and CA1 regions. Current source density analysis showed large sinks and sources in the dentate gyrus, similar to those evoked by perforant path stimulation. Gamma and theta frequencies were positively correlated. Putative interneurons in the dentate gyrus discharged at gamma frequency and were phase-locked to gamma waves. Bilateral entorhinal cortex lesions reduced hilar gamma activity, while a slower gamma pattern emerged in the CA3–CA1 network. The authors suggest that gamma oscillations arise from interactions between interneuron intrinsic properties and dentate gyrus network properties. They hypothesize that hilar gamma oscillations may be entrained by the entorhinal rhythm, while CA3–CA1 gamma oscillations are suppressed by the hilar region or entorhinal cortex. Gamma oscillations were observed in the dentate gyrus and CA1 region during spontaneous walking and paradoxical sleep. Gamma activity was larger during theta-associated behaviors. Gamma waves showed steep phase reversals across the granule cell layer. CSD maps showed distinct sink-source pairs for theta and gamma waves. Gamma waves were highly coherent along the longitudinal axis of the hilus but had low coherence in the transverse direction. Theta and gamma frequencies were correlated, with gamma frequency increasing as theta frequency decreased. Gamma oscillations were modulated by theta activity, with gamma power peaks occurring on the positive phase of theta waves. Interneurons in the hilus fired rhythmically and at higher rates during theta-associated behaviors. These interneurons were phase-locked to the ascending portion of gamma waves. Gamma oscillations were also correlated with unit activity, with neurons firing in rhythmic groups at theta frequency. Gamma oscillations were modulated by theta activity, with gamma power peaks occurring on the positive phase of theta waves. Entorhinal cortex lesions reduced gamma activity in the hilus but increased gamma activity in the CA1 region. Gamma oscillations in the CA1 region were slower and had lower amplitude. The authors suggest that the CA3–CA1 circuitry can oscillate at gamma frequency, and this propensity is expressed immediately after removal of the entorhinal cortex. The hilar gamma oscillator disappears or remains strongly attenuated. The study suggests that the entorhinal input plays a pivotal role in the generation of gamma oscillations. However, findings indicate that the entorhinal cortex does not simply impose its rhythm on a passive network in the dentate gyrus. Putative interneurons in the hilar region discharged coherently with gamma waves and contributed to extracellular currents. The rhythmic discharge of these neurons may result from network-driven excitation, intrinsic oscillatory properties, or both. The study also suggests that intrinsic oscillatory properties of interThis study examines gamma (40–100 Hz) oscillations in the hippocampus of awake rats. Gamma waves were highly coherent along the long axis of the dentate hilus but decreased in coherence in the CA3 and CA1 regions. Current source density analysis showed large sinks and sources in the dentate gyrus, similar to those evoked by perforant path stimulation. Gamma and theta frequencies were positively correlated. Putative interneurons in the dentate gyrus discharged at gamma frequency and were phase-locked to gamma waves. Bilateral entorhinal cortex lesions reduced hilar gamma activity, while a slower gamma pattern emerged in the CA3–CA1 network. The authors suggest that gamma oscillations arise from interactions between interneuron intrinsic properties and dentate gyrus network properties. They hypothesize that hilar gamma oscillations may be entrained by the entorhinal rhythm, while CA3–CA1 gamma oscillations are suppressed by the hilar region or entorhinal cortex. Gamma oscillations were observed in the dentate gyrus and CA1 region during spontaneous walking and paradoxical sleep. Gamma activity was larger during theta-associated behaviors. Gamma waves showed steep phase reversals across the granule cell layer. CSD maps showed distinct sink-source pairs for theta and gamma waves. Gamma waves were highly coherent along the longitudinal axis of the hilus but had low coherence in the transverse direction. Theta and gamma frequencies were correlated, with gamma frequency increasing as theta frequency decreased. Gamma oscillations were modulated by theta activity, with gamma power peaks occurring on the positive phase of theta waves. Interneurons in the hilus fired rhythmically and at higher rates during theta-associated behaviors. These interneurons were phase-locked to the ascending portion of gamma waves. Gamma oscillations were also correlated with unit activity, with neurons firing in rhythmic groups at theta frequency. Gamma oscillations were modulated by theta activity, with gamma power peaks occurring on the positive phase of theta waves. Entorhinal cortex lesions reduced gamma activity in the hilus but increased gamma activity in the CA1 region. Gamma oscillations in the CA1 region were slower and had lower amplitude. The authors suggest that the CA3–CA1 circuitry can oscillate at gamma frequency, and this propensity is expressed immediately after removal of the entorhinal cortex. The hilar gamma oscillator disappears or remains strongly attenuated. The study suggests that the entorhinal input plays a pivotal role in the generation of gamma oscillations. However, findings indicate that the entorhinal cortex does not simply impose its rhythm on a passive network in the dentate gyrus. Putative interneurons in the hilar region discharged coherently with gamma waves and contributed to extracellular currents. The rhythmic discharge of these neurons may result from network-driven excitation, intrinsic oscillatory properties, or both. The study also suggests that intrinsic oscillatory properties of inter